basic/socket-util: add hint to silence gcc's maybe-unitialized warning

basic/socket-util: add hint to silence gcc's maybe-unitialized warning

[59/1551] Compiling C object src/basic/libbasic.a.p/socket-util.c.o
../src/basic/socket-util.c: In function ‘socket_get_mtu’:
../src/basic/socket-util.c:1393:16: warning: ‘mtu’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 1393 |         *ret = (size_t) mtu;
      |                ^~~~~~~~~~~~