common: Drop image.h from common header

Move this uncommon header out of the common header.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/spl/spl_net.c b/common/spl/spl_net.c
index 30c050c..a9b6b07 100644
--- a/common/spl/spl_net.c
+++ b/common/spl/spl_net.c
@@ -9,6 +9,7 @@
 #include <common.h>
 #include <env.h>
 #include <errno.h>
+#include <image.h>
 #include <spl.h>
 #include <net.h>
 #include <linux/libfdt.h>