tests: trace: fix parameter type of aux callback

The function `git_win32__stack__set_aux_cb` expects the second parameter
to be a function callback of type `git_win32__stack__aux_cb_lookup`,
which expects a `size_t` parameter. In our test suite
trace::windows::stacktrace, we declare the callback with `unsigned int`
as parameter, though, causing a compiler warning.

Correct the parameter type to silence the warning.

tests/trace/windows/stacktrace.c

@@ -132,7 +132,7 @@ static void aux_cb_alloc__1(unsigned int *aux_id)
 	*aux_id = aux_counter++;
 }
 
-static void aux_cb_lookup__1(unsigned int aux_id, char *aux_msg, unsigned int aux_msg_len)
+static void aux_cb_lookup__1(unsigned int aux_id, char *aux_msg, size_t aux_msg_len)
 {
 	p_snprintf(aux_msg, aux_msg_len, "\tQQ%08x\n", aux_id);
 }