include/glib-compat.h defines a bunch of functions based on glib primitives,
and uses assert() without including assert.h.  Replace assert() with
g_assert() to make the file more self-contained, and to fix compilation
breakage after 28507a41.

Reported-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Tested-by: Laurent Desnogues <laurent.desnogues@gmail.com>