kbuild: fixdep: Check fstat(2) return value

Coverity has recently added a check that will find when we don't check
the return code from fstat(2).  Copy/paste the checking logic that
print_deps() has with an appropriate re-wording of the perror() message.

Signed-off-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Michal Marek <mmarek@suse.com>