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/fdt_support.c b/common/fdt_support.c
index 86de5b8..baf7924 100644
--- a/common/fdt_support.c
+++ b/common/fdt_support.c
@@ -7,6 +7,7 @@
  */
 
 #include <common.h>
+#include <env.h>
 #include <mapmem.h>
 #include <stdio_dev.h>
 #include <linux/ctype.h>