env: Move env_get() to env.h

Move env_get() over to the new header file.

Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/image.c b/common/image.c
index 719229c..4a578f3 100644
--- a/common/image.c
+++ b/common/image.c
@@ -8,6 +8,7 @@
 
 #ifndef USE_HOSTCC
 #include <common.h>
+#include <env.h>
 #include <watchdog.h>
 
 #ifdef CONFIG_SHOW_BOOT_PROGRESS