Put back inclusion of proc.h so that alpha kernels (at the very least)

will compile again. I can't quite see where this was a recursive inclusion.
We probably need to do something to fix the alpha, but let's not break it
in the interim- it's broken enough.

sys/sys/mutex.h

@@ -41,6 +41,7 @@
 #include <machine/bus.h>
 #include <machine/cpufunc.h>
 #include <machine/globals.h>
+#include <sys/proc.h> /* Needed for curproc. */
 #ifndef curproc
 struct proc;
 extern struct proc *curproc;