Auto merge of #2010 - tamird:if-namsiz, r=JohnTitor

Define IFNAMSIZ for Fuchsia

https://cs.opensource.google/fuchsia/fuchsia/+/master:zircon/third_party/ulib/musl/include/net/if.h;l=72;drc=bdf60afa9193c8a9190ee6bb6b5a0a11ad8876b6

Author: bors

src/fuchsia/mod.rs

@@ -1333,6 +1333,7 @@ pub const S_ISGID: ::c_int = 0x400;
 pub const S_ISVTX: ::c_int = 0x200;
 
 pub const IF_NAMESIZE: ::size_t = 16;
+pub const IFNAMSIZ: ::size_t = IF_NAMESIZE;
 
 pub const LOG_EMERG: ::c_int = 0;
 pub const LOG_ALERT: ::c_int = 1;