Fortran: Fix directory stat check for '.' [PR103560]

Fortran: Fix directory stat check for '.' [PR103560]

MinGW does not like a call to 'stat' for './' via gfc_do_check_include_dir.
Solution: Only append '/' when concatenating the path with the filename.

gcc/fortran/ChangeLog:

	PR fortran/103560
	* scanner.cc (add_path_to_list): Don't append '/' to the
	save include path.
	(open_included_file): Use '/' in concatenating path + file name.
	* module.cc (gzopen_included_file_1): Likewise.

gcc/testsuite/ChangeLog:

	PR fortran/103560
	* gfortran.dg/include_14.f90: Update dg-warning.
	* gfortran.dg/include_17.f90: Likewise.
	* gfortran.dg/include_18.f90: Likewise.
	* gfortran.dg/include_6.f90: Update dg-*.