nxstyle: header files that are not header files.

There is an issue with the graphics/nxglib/nxglib_fillrun.h header file (and probably others).

It is not really a header file. It is a "template" of a C file that is included with some definitions to make a complete C file. So it will NOT pass the tests for a header file.

See:

graphics/nxglib/lcd/nxglib_fillrectangle.c
graphics/nxglib/lcd/nxglib_filltrapezoid.c

It is not clear how this case should be handled. I believe we either have change the file name extension (but then it won't be checked at all) or change the extension to .c (which is misleading too).

Reference: PR #478