Fix Windows build.

While trying to resolve our CI issues, the Windows build broke with an
unused function error:

   src/test/test_switch_id.c:37:1: error: ‘unprivileged_port_range_start’
   defined but not used [-Werror=unused-function]

We solve this by moving the `#if !defined(_WIN32)` test above the
`unprivileged_port_range_start()` function defintion such that it is
included in its body.

This is an unreviewed commit.

See: tor#40275

src/test/test_switch_id.c

@@ -31,6 +31,8 @@ static const struct {
   { NULL, 0 }
 };
 
+#if !defined(_WIN32)
+
 /* Returns the first port that we think we can bind to without special
  * permissions. Usually this function returns 1024. */
 static uint16_t
@@ -67,8 +69,6 @@ unprivileged_port_range_start(void)
   return result;
 }
 
-#if !defined(_WIN32)
-
 #define PORT_TEST_RANGE_START 600
 #define PORT_TEST_RANGE_END   1024