sys_time: avoid compiler warnings

sys_time: avoid compiler warnings

sys/time uses #if/#else/#endif, and is therefore not subject
to the gcc optimization.  The #pragma GCC system_header must
occur up front.

* lib/sys_time.in.h (includes): Ensure gcc pragma is
unconditional, fixing regression from 2010-03-29.
Reported by Simon Josefsson.

Signed-off-by: Eric Blake <eblake@redhat.com>