Add -Werror-implicit-function-declaration

Add -Werror-implicit-function-declaration
This makes builds fail sooner if something is implicitly defined instead
of having to wait half an hour for it to fail at the linking stage.

Signed-off-by: Dave Jones <davej@redhat.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>