mark unused parameters in trivial compat functions

When a platform feature isn't available or in use, we sometimes
conditionally compile empty or trivial functions to turn these into
noops. We need to annotate their parameters so that -Wunused-parameters
won't complain about them.

Note that there are many more of these in compat/mingw.h, but we'll
leave them for now, as there's some trickery required to get the UNUSED
macro available there.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

exec-cmd.c

@@ -252,7 +252,7 @@ static const char *system_prefix(void)
  * This is called during initialization, but No work needs to be done here when
  * runtime prefix is not being used.
  */
-void git_resolve_executable_dir(const char *argv0)
+void git_resolve_executable_dir(const char *argv0 UNUSED)
 {
 }