ges: fix G_DECLARE_FINAL_TYPE -Werror with clang

ges: fix G_DECLARE_FINAL_TYPE -Werror with clang

Also fix wrong casing the `G_DECLARE` for GESDemux.

../subprojects/gst-editing-services/plugins/ges/gessrc.c:56:1: warning: unused function 'GES_SRC' [-Wunused-function]
G_DECLARE_FINAL_TYPE (GESSrc, ges_src, GES, SRC, GESBaseBin);
^
/usr/include/glib-2.0/gobject/gtype.h:1405:33: note: expanded from macro 'G_DECLARE_FINAL_TYPE'
static inline ModuleObjName * MODULE##_##OBJ_NAME (gpointer ptr) {                                     \
                              ^
<scratch space>:39:1: note: expanded from here
GES_SRC
^
../subprojects/gst-editing-services/plugins/ges/gessrc.c:56:1: warning: unused function 'GES_IS_SRC' [-Wunused-function]
/usr/include/glib-2.0/gobject/gtype.h:1407:26: note: expanded from macro 'G_DECLARE_FINAL_TYPE'
static inline gboolean MODULE##_IS_##OBJ_NAME (gpointer ptr) {                                         \
                        ^
<scratch space>:42:1: note: expanded from here
GES_IS_SRC
^

../subprojects/gst-editing-services/plugins/ges/gesdemux.c:50:1: warning: unused function 'GES_Demux' [-Wunused-function]
G_DECLARE_FINAL_TYPE (GESDemux, ges_demux, GES, Demux, GESBaseBin);
^
/usr/include/glib-2.0/gobject/gtype.h:1405:33: note: expanded from macro 'G_DECLARE_FINAL_TYPE'
static inline ModuleObjName * MODULE##_##OBJ_NAME (gpointer ptr) {                                     \
                              ^
<scratch space>:72:1: note: expanded from here
GES_Demux
^
../subprojects/gst-editing-services/plugins/ges/gesdemux.c:50:1: warning: unused function 'GES_IS_Demux' [-Wunused-function]
/usr/include/glib-2.0/gobject/gtype.h:1407:26: note: expanded from macro 'G_DECLARE_FINAL_TYPE'
static inline gboolean MODULE##_IS_##OBJ_NAME (gpointer ptr) {                                         \
                        ^
<scratch space>:75:1: note: expanded from here
GES_IS_Demux
^