We have a dedicated header file for wrappers to smooth over glib version
differences.  Move the g_poll() definition into glib-compat.h for
consistency.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Cc: qemu-trivial@nongnu.org