Fix a couple of ARI warnings in filestuff.c

[Joel Brobecker]

Hello,

I don't think there is any reason to avoid the use of gdb_stat.h
and gdb_dirent.h in common/filestuff.c, is there?

Tested on x86_64-linux, no regression. Will commit in a couple
of days if no objection.

Thank you,
-- 
Joel