bhyve: fix build with gcc48

Build with gcc 4.8 fails with:

bhyve/bhyve_monitor.c: In function 'bhyveMonitorIO':
bhyve/bhyve_monitor.c:51:18: error: missing initializer for field 'tv_sec' of 'const struct timespec' [-Werror=missing-field-initializers]
     const struct timespec zerowait = {};

Explicitly initialize zerowait to fix the build.

src/bhyve/bhyve_monitor.c

@@ -48,7 +48,7 @@ struct _bhyveMonitor {
 static void
 bhyveMonitorIO(int watch, int kq, int events ATTRIBUTE_UNUSED, void *opaque)
 {
-    const struct timespec zerowait = {};
+    const struct timespec zerowait = { 0, 0 };
     bhyveMonitorPtr mon = opaque;
     struct kevent kev;
     int rc, status;