Amendment to r28508 (Make Blender malloc be 64 bit ready)

- one function missed in header causes compile problems (intern/guardedalloc/intern/mallocn.c:352: error: conflicting types for ‘MEM_mapallocN’)

intern/guardedalloc/MEM_guardedalloc.h

@@ -105,7 +105,7 @@ extern "C" {
 	/** Same as callocN, clears memory and uses mmap (disk cached) if supported.
 		Can be free'd with MEM_freeN as usual.
 		* */
-	void *MEM_mapallocN(unsigned int len, const char * str);
+	void *MEM_mapallocN(size_t len, const char * str);
 
 	/** Print a list of the names and sizes of all allocated memory
 	 * blocks. as a python dict for easy investigation */