dispatch: include time.h on windows

time.h is needed on Windows to ensure that we have a definition of
`timespec`.  This is needed when building the swift SDK overlay.  It
seems that when cross-compiling, we were getting lucky with the
`timespec` definition being provided by some other header being
included.  This repairs the build of libdispatch's swift SDK overlay on
Windows targeting Windows.

Author: compnerd

dispatch/dispatch.h

@@ -42,6 +42,9 @@
 #include <unistd.h>
 #endif
 #include <fcntl.h>
+#if defined(_WIN32)
+#include <time.h>
+#endif
 
 #if (defined(__linux__) || defined(__FreeBSD__)) && defined(__has_feature)
 #if __has_feature(modules)