cmake: Fix inconsistent endif() argument

The argument in the corresponding if() was changed in
commit ae76a38. CMake emits a warning.
Modern CMake style guides advice on ommitting endif()
arguments.

CMakeLists.txt

@@ -66,7 +66,7 @@ if(CPPDAP_BUILD_TESTS AND NOT CPPDAP_USE_EXTERNAL_GTEST_PACKAGE)
         message(WARNING "Run: `git submodule update --init` to build tests.")
         set(CPPDAP_BUILD_TESTS OFF)
     endif()
-endif(CPPDAP_BUILD_TESTS)
+endif()
 
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 # JSON library