spl, spl_nor: fix compiler warning

executing "tools/buildman/buildman mpc5xx" drops this warning:

common/spl/spl_nor.c: In function 'spl_nor_load_image':
common/spl/spl_nor.c:26:10: warning: assignment discards 'const' qualifier from pointer target type [enabled by default]

fix this.

Signed-off-by: Heiko Schocher <hs@denx.de>
diff --git a/common/spl/spl_nor.c b/common/spl/spl_nor.c
index 2c0e8e0..c2fee01 100644
--- a/common/spl/spl_nor.c
+++ b/common/spl/spl_nor.c
@@ -17,7 +17,7 @@
 
 #ifdef CONFIG_SPL_OS_BOOT
 	if (!spl_start_uboot()) {
-		struct image_header *header;
+		const struct image_header *header;
 
 		/*
 		 * Load Linux from its location in NOR flash to its defined