MINOR: time: export the global_now variable

This is the process-wide monotonic time that is used to update each
thread's own time. It may be required at a few places where a strictly
monotonic clock is required such as freq_ctr. It will be have to be
backported as a dependency of a forthcoming fix.
diff --git a/src/time.c b/src/time.c
index a665a85..d5344d2 100644
--- a/src/time.c
+++ b/src/time.c
@@ -28,7 +28,7 @@
 THREAD_LOCAL struct timeval after_poll;      /* system date after leaving poll() */
 
 static THREAD_LOCAL struct timeval tv_offset;  /* per-thread time ofsset relative to global time */
-static volatile unsigned long long global_now; /* common date between all threads (32:32) */
+volatile unsigned long long global_now;      /* common date between all threads (32:32) */
 
 static THREAD_LOCAL unsigned int iso_time_sec;     /* last iso time value for this thread */
 static THREAD_LOCAL char         iso_time_str[34]; /* ISO time representation of gettimeofday() */