escape_backslashes should be used unconditionally

[Artoria2e5]

We currently have

gpp/src/gpp.c

Lines 3137 to 3141 in 8e6fe69

	#ifdef WIN_NT
	escape_backslashes(filename,&escapedfilename);
	#else
	escapedfilename = filename;
	#endif

with the justification

gpp/src/gpp.c

Lines 3147 to 3149 in 8e6fe69

	/* Under windows, files can have backslashes in them.
	These should be escaped.
	*/

.

Other platforms (mainly Unix-like) can have backslashes in the filename too -- they may even be more liberal at that and allow newlines and other fun stuff.