uapi: stddef.h: Fix header guard location

The #endif for the header guard wasn't at the end of the header. This
was harmless since the define that escaped was already testing for its
own redefinition. Regardless, move the #endif to the correct place.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Fixes: c8248faf ("Compiler Attributes: counted_by: Adjust name and identifier expansion")
Link: https://lore.kernel.org/r/b1f5081e-339d-421d-81b2-cbb94e1f6f5f@p183


Co-developed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>