buffer: Remove const from cogl_is_buffer (const void *)

None of the other cogl_is_blah functions have a const pointer so this
is just for consistency. It helps if someone is trying to have an
array of type-check function pointers to determine the Cogl object
type because in that case all of the functions would have to have the
same prototype.

Reviewed-by: Robert Bragg <robert@linux.intel.com>

cogl/cogl-buffer.c

@@ -85,7 +85,7 @@ _cogl_buffer_register_buffer_type (const CoglObjectClass *klass)
 }
 
 gboolean
-cogl_is_buffer (const void *object)
+cogl_is_buffer (void *object)
 {
   const CoglHandleObject *obj = object;
   GSList *l;

cogl/cogl-buffer.h

@@ -75,7 +75,7 @@ typedef struct _CoglBuffer CoglBuffer;
  * Stability: unstable
  */
 gboolean
-cogl_is_buffer (const void *object);
+cogl_is_buffer (void *object);
 
 /**
  * cogl_buffer_get_size: