env: Move env_set_ulong() to env.h

Move env_set_ulong() over to the new header file.

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

drivers/bootcount/bootcount_env.c

@@ -5,6 +5,7 @@
  */
 
 #include <common.h>
+#include <env.h>
 #include <environment.h>
 
 void bootcount_store(ulong a)