Give a error when running out of iomem areas.

The limit of iomem areas is quite low. Without the
debug print, it is quite hard to figure out why more
devices are not getting registered.
Signed-off-by: NRiku Voipio <riku.voipio@nokia.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

exec.c

@@ -2919,7 +2919,7 @@ static int get_free_io_mem_idx(void)
             io_mem_used[i] = 1;
             return i;
         }
-
+    fprintf(stderr, "RAN out out io_mem_idx, max %d !\n", IO_MEM_NB_ENTRIES);
     return -1;
 }