rt: alloca is spelled differently on win32

src/rt/rust_uv.cpp

@@ -98,7 +98,11 @@ rust_uv_loop_delete(uv_loop_t* loop) {
     // from a malloc with no backtrace.
     //
     // This pads our stack with some extra space before deleting the loop
+#ifndef __WIN32__
     alloca(512);
+#else
+    _alloca(512);
+#endif
     uv_loop_delete(loop);
 }