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/lib/smbios.c b/lib/smbios.c
index e8ee55c..7b74971 100644
--- a/lib/smbios.c
+++ b/lib/smbios.c
@@ -6,6 +6,7 @@
  */
 
 #include <common.h>
+#include <env.h>
 #include <mapmem.h>
 #include <smbios.h>
 #include <tables_csum.h>