thc-ipv6: fix compilation with GCC 15

thc-ipv6: fix compilation with GCC 15

GCC 15 defaults to C23, where an empty parameter list "()" means
"(void)" instead of an unspecified argument list. This broke thc-ipv6 in
two ways:

 - thc-ipv6.h declared thc_open_ipv6() with an empty argument list, so
   its prototype became "int(void)" and conflicted with the actual
   definition "int thc_open_ipv6(char *interface)" and all of its
   callers:

     thc-ipv6-lib.c:127:36: error: too many arguments to function
     'thc_open_ipv6'; expected 0, have 1
     thc-ipv6-lib.c:2555:5: error: conflicting types for 'thc_open_ipv6';
     have 'int(char *)'

 - The alarming() SIGALRM handlers in detect_sniffer6.c and thcping6.c
   were declared with "()", so their type became "void (*)(void)" and no
   longer matched the "void (*)(int)" expected by signal():

     detect_sniffer6.c:140:19: error: passing argument 2 of 'signal'
     from incompatible pointer type [-Wincompatible-pointer-types]

Add patches giving the prototype and the signal handlers their proper
signatures.

Assisted-by: Claude:claude-opus-4-8
Link: https://github.com/openwrt/openwrt/pull/23956
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>