Revert "Change preferred default clocksource to gettimeofday()"

This reverts commit f7161017b75f5d3526147d42d3b583973922c444.

clock_gettime() is monotonic, so a superior clock source even
if it is slower.
diff --git a/os/os.h b/os/os.h
index 1d3a750..2e4764e 100644
--- a/os/os.h
+++ b/os/os.h
@@ -135,7 +135,7 @@
 #endif
 
 #ifndef FIO_PREFERRED_CLOCK_SOURCE
-#define FIO_PREFERRED_CLOCK_SOURCE	CS_GTOD
+#define FIO_PREFERRED_CLOCK_SOURCE	CS_CGETTIME
 #endif
 
 #ifndef FIO_MAX_JOBS