build: remove extra include path for system vixl

As far as I know the only pkg-config file provided by Vixl is the one generated by Meson when applying my yet to be merged patch.

That extra include path was needed because I mistakenly thought that adding `vixl` as an include subdirectory was not necessary, but I fixed it in my latest revision - more details here: Linaro/vixl#7 (comment)

The fix already landed in Debian and Ubuntu, that as far as I know are the only Linux distros that ship my patch, so manually adding that include directory shouldn't be necessary anymore

CMakeLists.txt

@@ -134,7 +134,6 @@ endif()
 if (DYNARMIC_NO_BUNDLED_VIXL AND ARCHITECTURE STREQUAL "arm64")
     find_package(PkgConfig REQUIRED)
     pkg_check_modules(vixl REQUIRED IMPORTED_TARGET vixl)
-    target_include_directories(PkgConfig::vixl INTERFACE "${vixl_INCLUDE_DIRS}/vixl")
     add_library(vixl ALIAS PkgConfig::vixl)
 endif()