mkimage: Update map_to_sysmem() to match its prototype

Update the version of this function in mkimage so that it uses a const
pointer, as is done in the mapmem.h header file.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/mkimage.h b/tools/mkimage.h
index d92a3ff..15741f2 100644
--- a/tools/mkimage.h
+++ b/tools/mkimage.h
@@ -37,7 +37,7 @@
 	return (void *)(uintptr_t)paddr;
 }
 
-static inline ulong map_to_sysmem(void *ptr)
+static inline ulong map_to_sysmem(const void *ptr)
 {
 	return (ulong)(uintptr_t)ptr;
 }