Update msr.c

Silences even more warnings at compile time.

src/ss/msr.c

@@ -4,6 +4,8 @@
 #include <unistd.h>
 #include <fcntl.h>
 #include <math.h>
+#include "stdlib.h"
+#include "string.h"
 #include "zenmonitor.h"
 #include "msr.h"