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/splash.c b/common/splash.c
index 79d5057..e15cc84 100644
--- a/common/splash.c
+++ b/common/splash.c
@@ -21,6 +21,7 @@
  */
 
 #include <common.h>
+#include <env.h>
 #include <splash.h>
 #include <lcd.h>