RISC OS: Include core bitmap header from RO bitmap header.

Squash warnings about enum declared inside parameter list.

frontends/riscos/bitmap.c

@@ -44,7 +44,6 @@
 #include "utils/log.h"
 #include "utils/messages.h"
 #include "netsurf/plotters.h"
-#include "netsurf/bitmap.h"
 #include "netsurf/content.h"
 
 #include "riscos/gui.h"

frontends/riscos/bitmap.h

@@ -19,6 +19,8 @@
 #ifndef _NETSURF_RISCOS_BITMAP_H_
 #define _NETSURF_RISCOS_BITMAP_H_
 
+#include "netsurf/bitmap.h"
+
 struct osspriteop_area;
 struct osspriteop_header;
 struct hlcache_handle;