signal.c: Fix undefined reset_sigmask use on Emscripten

3c47114 added a call to `reset_sigmask`
but the guard condition didn't match with the condition used to define
`reset_sigmask`.

signal.c

@@ -403,7 +403,7 @@ interrupt_init(int argc, VALUE *argv, VALUE self)
 }
 
 void rb_malloc_info_show_results(void); /* gc.c */
-#ifdef POSIX_SIGNAL
+#if defined(USE_SIGALTSTACK) || defined(_WIN32)
 static void reset_sigmask(int sig);
 #endif
 
@@ -416,7 +416,7 @@ ruby_default_signal(int sig)
     rb_malloc_info_show_results();
 
     signal(sig, SIG_DFL);
-#ifdef POSIX_SIGNAL
+#if defined(USE_SIGALTSTACK) || defined(_WIN32)
     reset_sigmask(sig);
 #endif
     raise(sig);