Add a missing #include. I suspect this used to get included via some header

pollution that was cleaned up recently, and this file got missed in the
cleanup because it's not attached to the build unless you specifically
request this device in a custom kernel config.

sys/arm/ti/am335x/am335x_dmtpps.c

@@ -50,6 +50,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/kernel.h>
 #include <sys/module.h>
 #include <sys/malloc.h>
+#include <sys/mutex.h>
 #include <sys/rman.h>
 #include <sys/taskqueue.h>
 #include <sys/timepps.h>