C++ suffix on C-file

[joakimono]

When attempting to build this library (v2.4.1) without access to a C++ compiler,
CMake gets confused about this file: Test/FMI2/fmi2_import_xml_test.cc.

If I change in CMakeLists.txt project (FMILibrary LANGUAGES C) and rename the file above to a .c extension, it compiles fine.

I notice that for the next major release, FMI 3 tests are using C++, so perhaps not so relevant going forward.