FindMatlab: in matlab_add_mex use the correct version file

Before this modification, the c_mexapi_version.c file was added to
all mex libraries. However, if the C language was not enabled
in the CMake project configuration, the c_mexapi_version.c file
was ignored, creating linking errors in Windows and macOS.
This commit ensures that in the case only the CXX languages is enabled,
the correct version is passed.

Fixes: #19382