Handle headers not included in project files.

[ludviggunne]

When project files are provided, --file-filter=... is used for the input file.
This results in these types of errors when running on header files:

cppcheck: error: could not find any files matching the filter: ...

[danmar]

Some possible options could be :

• if foo.h is checked then try to lookup foo.cpp in the project file and use those options
• add default cppcheck options in run-cppcheck.config used when checking headers