<sys/lock.h> is a prerequisite for <sys/mutex.h>, so include the former

here before the latter instead of depending on namespace pollution in
<sys/mumble.h> or on accidentally placed includes in netsmb/*.c.

sys/netsmb/smb_subr.h

@@ -76,6 +76,7 @@ void m_dumpm(struct mbuf *m);
  * Compatibility wrappers for simple locks
  */
 
+#include <sys/lock.h>
 #include <sys/mutex.h>
 
 #define	smb_slock			mtx