Fix safety of windows uwp functions

These functions were changed to be safe in
rust-lang#127763, but this particular UWP
version was missed. Otherwise this causes unnecessary unsafe block
warnings/errors.

std/src/sys/pal/windows/stack_overflow_uwp.rs

@@ -1,4 +1,4 @@
 #![cfg_attr(test, allow(dead_code))]
 
-pub unsafe fn reserve_stack() {}
-pub unsafe fn init() {}
+pub fn reserve_stack() {}
+pub fn init() {}