configfile.c: don't include system headers directly

git-compat-util.h may define various values that affect the
interpretation of system headers.  In most places we include cgit.h
first, which pulls in git-compat-util.h, but this file does not depend
on anything else in CGit, so use git-compat-util.h directly.

Signed-off-by: John Keeping <john@keeping.me.uk>