Fix build failure with GCC4.6

http://projectm.svn.sourceforge.net/viewvc/projectm?view=revision&revision=1355

--- trunk/src/projectM-libvisual/actor_projectM.cpp	2009/02/20 19:07:44	1210
+++ trunk/src/projectM-libvisual/actor_projectM.cpp	2011/05/06 12:04:58	1355
@@ -209,7 +209,7 @@
  * we aren't with projectm, so just ignore :) */
 extern "C" VisPalette *lv_projectm_palette (VisPluginData *plugin)
 {
-	return NULL;
+	return (VisPalette *) NULL;
 }
 
 /* This is where the real rendering happens! This function is what we call, many times