[BUILD] Build break in OLTP_FILE tests

[marcalff]

Using CMake -DBUILD_TESTING=ON -DWITH_OTLP_FILE=ON

The build fails with:

[ 83%] Building CXX object exporters/otlp/CMakeFiles/otlp_file_client_test.dir/test/otlp_file_client_test.cc.o
/.../opentelemetry-cpp/exporters/otlp/test/otlp_file_client_test.cc:23:10: fatal error: nlohmann/json.hpp: No such file or directory
   23 | #include "nlohmann/json.hpp"
      |          ^~~~~~~~~~~~~~~~~~~
compilation terminated.
make[2]: *** [exporters/otlp/CMakeFiles/otlp_file_client_test.dir/build.make:76: exporters/otlp/CMakeFiles/otlp_file_client_test.dir/test/otlp_file_client_test.cc.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:6643: exporters/otlp/CMakeFiles/otlp_file_client_test.dir/all] Error 2
make: *** [Makefile:146: all] Error 2

Note that nlohmann_json is not installed under /usr/include, but in a different location, so it is not part of the default include path.