shmsink: Fix compilation with latest GLib

shmsink: Fix compilation with latest GLib

g_object_ref() forwards its arguments type.

gstshmsink.c: In function ‘gst_shm_sink_allocator_alloc_locked’:
/usr/include/glib-2.0/gobject/gobject.h:512:32: error: passing argument 3 of ‘gst_memory_init’ from incompatible pointer type [-Werror=incompatible-pointer-types]
 #define g_object_ref(Obj)      ((__typeof__(Obj)) (g_object_ref) (Obj))
                                ^
gstshmsink.c:292:45: note: in expansion of macro ‘g_object_ref’
     gst_memory_init (memory, params->flags, g_object_ref (self), NULL,
                                             ^~~~~~~~~~~~