core: avoid "-Wmaybe-uninitialized" warning in update_system_hostname() with LTO

When building without more-assertions and LTO, the compiler might think
that "wait" is uninitialized. Avoid the warning.

Initializing a variable is not a great solution either, because
potentially it could hide an actual bug. But it still seems to be
best.

    src/nm-policy.c: In function update_system_hostname:
    src/nm-policy.c:909: warning: wait may be used uninitialized in this function [-Wmaybe-uninitialized]
      909 |                     if (wait) {
          |
    src/nm-policy.c:901: note: wait was declared here
      901 |                     gboolean    wait;
          |

src/nm-policy.c

@@ -898,7 +898,7 @@ update_system_hostname(NMPolicy *self, const char *msg)
 
                 if (info->from_dns && info->ip_x[IS_IPv4]) {
                     const char *result;
-                    gboolean    wait;
+                    gboolean    wait = FALSE;
 
                     result =
                         nm_device_get_hostname_from_dns_lookup(info->device, addr_family, &wait);