Auto merge of #2801 - devnexen:macos_ipv6_bound_if, r=JohnTitor

apple adding `IPV6_BOUND_IF` socket constant.

closes #2793

Author: bors

libc-test/semver/apple.txt

@@ -525,6 +525,7 @@ IPTOS_ECN_ECT0
 IPTOS_ECN_ECT1
 IPTOS_ECN_MASK
 IPTOS_ECN_NOTECT
+IPV6_BOUND_IF
 IPV6_CHECKSUM
 IPV6_HOPLIMIT
 IPV6_JOIN_GROUP

src/unix/bsd/apple/mod.rs

@@ -3561,6 +3561,7 @@ pub const IP_ADD_SOURCE_MEMBERSHIP: ::c_int = 70;
 pub const IP_DROP_SOURCE_MEMBERSHIP: ::c_int = 71;
 pub const IP_BLOCK_SOURCE: ::c_int = 72;
 pub const IP_UNBLOCK_SOURCE: ::c_int = 73;
+pub const IPV6_BOUND_IF: ::c_int = 125;
 
 pub const TCP_NOPUSH: ::c_int = 4;
 pub const TCP_NOOPT: ::c_int = 8;