bsd: add missing os.IFNAMESIZE

- based on system API value IF_NAMESIZE
- unblocks `zig test lib/std/std.zig`

lib/std/c/dragonfly.zig

@@ -838,6 +838,8 @@ pub const EAI = enum(c_int) {
     _,
 };
 
+pub const IFNAMESIZE = 16;
+
 pub const AI = struct {
     pub const PASSIVE = 0x00000001;
     pub const CANONNAME = 0x00000002;

lib/std/c/netbsd.zig

@@ -483,6 +483,8 @@ pub const sockaddr = extern struct {
     };
 };
 
+pub const IFNAMESIZE = 16;
+
 pub const AI = struct {
     /// get address to use bind()
     pub const PASSIVE = 0x00000001;

lib/std/c/openbsd.zig

@@ -388,6 +388,8 @@ pub const sockaddr = extern struct {
     };
 };
 
+pub const IFNAMESIZE = 16;
+
 pub const AI = struct {
     /// get address to use bind()
     pub const PASSIVE = 1;