Fix build when cpp_reserved_words_union_typedef.hh is not ready (#3079)

Here is the build log on FreeBSD.
```
[ 95% 96/101] cd /wrkdirs/usr/ports/devel/avro-cpp/work/.build && /wrkdirs/usr/ports/devel/avro-cpp/work/.build/avrogencpp -p - -i /wrkdirs/usr/ports/devel/avro-cpp/work/avro-cpp-1.12.0/jsonschemas/cpp_reserved_words -o cpp_reserved_words.hh -n cppres
[ 96% 97/101] /usr/local/libexec/ccache/c++ -DAVRO_VERSION=\"1.12.0\" -DFMT_HEADER_ONLY=1 -I/wrkdirs/usr/ports/devel/avro-cpp/work/avro-cpp-1.12.0/include/avro -I/wrkdirs/usr/ports/devel/avro-cpp/work/.build -I/wrkdirs/usr/ports/devel/avro-cpp/work/avro-cpp-1.12.0/include -isystem /usr/local/include -O2 -pipe  -fstack-protector-strong -fno-strict-aliasing   -std=c++17 -O2 -pipe  -fstack-protector-strong -fno-strict-aliasing   -std=c++17  -DNDEBUG -std=gnu++17 -MD -MT CMakeFiles/AvrogencppTestReservedWords.dir/test/AvrogencppTestReservedWords.cc.o -MF CMakeFiles/AvrogencppTestReservedWords.dir/test/AvrogencppTestReservedWords.cc.o.d -o CMakeFiles/AvrogencppTestReservedWords.dir/test/AvrogencppTestReservedWords.cc.o -c /wrkdirs/usr/ports/devel/avro-cpp/work/avro-cpp-1.12.0/test/AvrogencppTestReservedWords.cc
FAILED: CMakeFiles/AvrogencppTestReservedWords.dir/test/AvrogencppTestReservedWords.cc.o
/usr/local/libexec/ccache/c++ -DAVRO_VERSION=\"1.12.0\" -DFMT_HEADER_ONLY=1 -I/wrkdirs/usr/ports/devel/avro-cpp/work/avro-cpp-1.12.0/include/avro -I/wrkdirs/usr/ports/devel/avro-cpp/work/.build -I/wrkdirs/usr/ports/devel/avro-cpp/work/avro-cpp-1.12.0/include -isystem /usr/local/include -O2 -pipe  -fstack-protector-strong -fno-strict-aliasing   -std=c++17 -O2 -pipe  -fstack-protector-strong -fno-strict-aliasing   -std=c++17  -DNDEBUG -std=gnu++17 -MD -MT CMakeFiles/AvrogencppTestReservedWords.dir/test/AvrogencppTestReservedWords.cc.o -MF CMakeFiles/AvrogencppTestReservedWords.dir/test/AvrogencppTestReservedWords.cc.o.d -o CMakeFiles/AvrogencppTestReservedWords.dir/test/AvrogencppTestReservedWords.cc.o -c /wrkdirs/usr/ports/devel/avro-cpp/work/avro-cpp-1.12.0/test/AvrogencppTestReservedWords.cc
/wrkdirs/usr/ports/devel/avro-cpp/work/avro-cpp-1.12.0/test/AvrogencppTestReservedWords.cc:19:10: fatal error: 'cpp_reserved_words_union_typedef.hh' file not found
   19 | #include "cpp_reserved_words_union_typedef.hh"
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
ninja: build stopped: subcommand failed.
```

lang/c++/CMakeLists.txt

@@ -218,6 +218,8 @@ unittest (AvrogencppTestReservedWords)
 unittest (CommonsSchemasTests)
 
 add_dependencies (AvrogencppTestReservedWords cpp_reserved_words_hh)
+add_dependencies (AvrogencppTestReservedWords cpp_reserved_words_hh
+    cpp_reserved_words_union_typedef_hh)
 
 add_dependencies (AvrogencppTests bigrecord_hh bigrecord_r_hh bigrecord2_hh
     tweet_hh