Add missing header to avoid:

| 1472659610.016355: ../git/pthread_stop_world.c: In function 'GC_brief_async_signal_safe_sleep':
| 1472659610.0540252: ../git/pthread_stop_world.c:397:22: error: storage size of 'tv' isn't known
| 1472659610.0540252:        struct timeval tv;
| 1472659610.0540252:                       ^~
| 1472659610.054099: ../git/pthread_stop_world.c:397:22: warning: unused variable 'tv' [-Wunused-variable]
| 1472659610.054099:        struct timeval tv;
| 1472659610.054099:                       ^~
| 1472659610.054099: Makefile:1530: recipe for target 'pthread_stop_world.lo' failed

in musl builds.

Upstream-Status: Pending

Index: git/pthread_stop_world.c
===================================================================
--- git.orig/pthread_stop_world.c
+++ git/pthread_stop_world.c
@@ -45,6 +45,7 @@
 #include <semaphore.h>
 #include <errno.h>
 #include <unistd.h>
+#include <sys/time.h>
 #include "atomic_ops.h"
 
 /* It's safe to call original pthread_sigmask() here.   */