clutter: Fix compilation error in GL examples

clutter: Fix compilation error in GL examples

Include gst/gl.h instead of specific headers to prevent such problems also in
the future.

In file included from ../../../../gst-libs/gst/gl/gl.h:47:0,
                 from ../../../../gst-libs/gst/gl/gstglbasememory.h:137,
                 from ../../../../gst-libs/gst/gl/gstglmemory.h:29,
                 from cluttershare.c:39:
../../../../gst-libs/gst/gl/gstglmemorypbo.h:51:20: error: field ‘mem’ has incomplete type
   GstGLMemory      mem;
                    ^
../../../../gst-libs/gst/gl/gstglmemorypbo.h:124:24: error: field ‘parent’ has incomplete type
   GstGLMemoryAllocator parent;
                        ^
../../../../gst-libs/gst/gl/gstglmemorypbo.h:134:29: error: field ‘parent_class’ has incomplete type
   GstGLMemoryAllocatorClass parent_class;
                             ^