slirp: Fix compiler warning for w64

Casting a pointer to an integer value must use uintptr_t or intptr_t
(not long) for portable code. MinGW-w64 requires this because
sizeof(long) != sizeof(void *) for w64 hosts, so casting to long
raises a compiler warning.

I use uintptr_t instead of intptr_t because changing the sign does not
matter here and casting pointers to unsigned values seems more
reasonable (the unsigned value is a non negative offset.

Cc: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
