dhcpv6-ia: add missing limits header

dhcpv6-ia: add missing limits header

Fix the following build error by including <limits.h>:
src/dhcpv6-ia.c: In function 'handle_addrlist_change':
src/dhcpv6-ia.c:495:53: error: 'INT_MAX' undeclared (first use in this function)
  495 |                                         a->fr_cnt = INT_MAX;
      |                                                     ^~~~~~~
src/dhcpv6-ia.c:33:1: note: 'INT_MAX' is defined in header '<limits.h>'; did you forget to '#include <limits.h>'?
   32 | #include <libubox/md5.h>
  +++ |+#include <limits.h>
   33 |
src/dhcpv6-ia.c:495:53: note: each undeclared identifier is reported only once for each function it appears in
  495 |                                         a->fr_cnt = INT_MAX;
      |                                                     ^~~~~~~

Fixes: 7136fbe390a5 ("dhcpv6-ia: split statefile handling to separate file")
Signed-off-by: Álvaro Fernández Rojas <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/311