Actually block in a windows cvar

Turns out with an argument of 0 the function always returns immediately!

Closes #11003

src/libstd/unstable/mutex.rs

@@ -286,7 +286,7 @@ mod imp {
 
     pub unsafe fn wait(cond: *c_void, m: *c_void) {
         unlock(m);
-        WaitForSingleObject(cond as HANDLE, 0);
+        WaitForSingleObject(cond as HANDLE, libc::INFINITE);
         lock(m);
     }