[ccache] use_relative_paths_in_depfile not called in preprocessor mode

[Karanam Suryanarayana Rao Venkata Krishna]

Hello,
cache hit(preprocessed) puts the .d file from the workspace as-is into the
cache, thus, if such a compilation resulted in absolute paths in the .d
file(in workspace), then, the cache will also contain .d files with
absolute paths, resulting in troubles later on(from another users workspace)

I mean, https://github.com/ccache/ccache/blob/3.3-maint/ccache.c#L1969
should precede a function call use_relative_paths_in_depfile(output_dep);
just the way it is on
https://github.com/ccache/ccache/blob/3.3-maint/ccache.c#L1307

Please comment.

best regards,
Venkat.