Add missing function declarations

They are absent even in upstream, so I decided against copying to the
local copy of libogc. And a couple of function prototypes doesn't seem
to warrant a new header file.

gfx/drivers/gx_gfx.c

@@ -74,6 +74,9 @@
   }
 #endif
 
+void VIDEO_SetTrapFilter(bool enable);
+void VIDEO_SetGamma(int gamma);
+
 enum
 {
    GX_RESOLUTIONS_DEFAULT = 0,