include config.h in all sources

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

src/mapper.c

@@ -21,6 +21,7 @@
  * Maps directory and song objects to file system paths.
  */
 
+#include "config.h"
 #include "mapper.h"
 #include "directory.h"
 #include "song.h"