env: Move env_set() to env.h

Move env_set() over to the new header file.

Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Simon Glass <sjg@chromium.org>

include/env_flags.h

@@ -146,6 +146,7 @@ int env_flags_validate_env_set_params(char *name, char *const val[], int count);
 
 #else /* !USE_HOSTCC */
 
+#include <env.h>
 #include <search.h>
 
 /*