Fix building on 6.12 due to missing definition of rdtsc().

Something else must have been indirectly pulling in asm/msr.h.
Explicitly add it to the includes.

This should be fine for kernels newer than 3.x. 3.x and earlier
never had a rdtsc() function by the looks of it, so I'm not sure
they would even build today. (That could be a separate fix if
needed in the future.)

Fixes #4.

pvclock.c

@@ -5,6 +5,7 @@
  */
 #include <linux/sched.h>
 #include <linux/sched/clock.h>
+#include <asm/msr.h>
 
 #include "pvclock.h"