srt: Fix GCC function type error

gstsrtobject.c: In function ‘gst_srt_object_close’:
gstsrtobject.c:1036:7: error: function called through a non-compatible type [-Werror]
       (GDestroyNotify) g_closure_unref);
/usr/include/glib-2.0/glib/gmem.h:121:8: note: in definition of macro ‘g_clear_pointer’
       (destroy) (_ptr);                                                        \
        ^~~~~~~
gstsrtobject.c:1038:7: error: function called through a non-compatible type [-Werror]
       (GDestroyNotify) g_closure_unref);
/usr/include/glib-2.0/glib/gmem.h:121:8: note: in definition of macro ‘g_clear_pointer’
       (destroy) (_ptr);                                                        \
        ^~~~~~~

Arch Linux
gcc 8.2.1 20181127
glib 2.58.2