Fix missing include in win32/globals.c

This previously errored with:

win32\globals.c(66): error C2220: the following warning is treated as an error
win32\globals.c(66): warning C4013: 'php_win32_signal_ctrl_handler_request_shutdown' undefined; assuming extern returning int

This only errors on master because of 2473f57ba (thanks to Niels for
that info!).

Closes GH-18508

win32/globals.c

@@ -17,6 +17,7 @@
 #include "php.h"
 #include "php_win32_globals.h"
 #include "syslog.h"
+#include "signal.h"
 
 #ifdef ZTS
 PHPAPI int php_win32_core_globals_id;