progs/perf: offsetof() should be defined in stddef.h, include it

If this breaks mingw, feel free to revert this.
diff --git a/progs/perf/common.h b/progs/perf/common.h
index fc49bbe..6ea1740 100644
--- a/progs/perf/common.h
+++ b/progs/perf/common.h
@@ -24,6 +24,9 @@
 #define COMMON_H
 
 
+#include <stddef.h> /* for offsetof() */
+
+
 typedef void (*PerfRateFunc)(unsigned count);