unbreak OpenBSD after #3714 by properly define RTF_FMASK

Author: semarie

src/unix/bsd/netbsdlike/openbsd/mod.rs

@@ -1896,7 +1896,9 @@ pub const RTF_LOCAL: ::c_int = 0x200000;
 pub const RTF_BROADCAST: ::c_int = 0x400000;
 pub const RTF_CONNECTED: ::c_int = 0x800000;
 pub const RTF_BFD: ::c_int = 0x1000000;
-pub const RTF_FMASK: ::c_int = b'\\' as _;
+pub const RTF_FMASK: ::c_int = ::RTF_LLINFO | ::RTF_PROTO1 | ::RTF_PROTO2
+  | ::RTF_PROTO3 | ::RTF_BLACKHOLE | ::RTF_REJECT | ::RTF_STATIC
+  | ::RTF_MPLS | ::RTF_BFD;
 
 pub const RTM_VERSION: ::c_int = 5;
 pub const RTM_RESOLVE: ::c_int = 0xb;