escape_backslashes should be used unconditionally

[Artoria2e5]

We currently have https://github.com/logological/gpp/blob/8e6fe69819d1c2cb2fd5cadfd3193ff99993264d/src/gpp.c#L3137-L3141 with the justification https://github.com/logological/gpp/blob/8e6fe69819d1c2cb2fd5cadfd3193ff99993264d/src/gpp.c#L3147-L3149.

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.