linux musl adding PIDFD_NONBLOCK constant.

closes rust-lang#3002

Author: devnexen

libc-test/semver/linux-musl.txt

@@ -20,6 +20,7 @@ LIO_WRITE
 PF_IB
 PF_MPLS
 PF_XDP
+PIDFD_NONBLOCK
 PR_SET_VMA
 PR_SET_VMA_ANON_NAME
 adjtimex
@@ -54,4 +55,4 @@ asctime_r
 strftime
 strptime
 dirname
-basename
+basename

src/unix/linux_like/linux/musl/mod.rs

@@ -590,6 +590,8 @@ pub const EFD_NONBLOCK: ::c_int = ::O_NONBLOCK;
 
 pub const SFD_NONBLOCK: ::c_int = ::O_NONBLOCK;
 
+pub const PIDFD_NONBLOCK: ::c_uint = O_NONBLOCK as ::c_uint;
+
 pub const TCSANOW: ::c_int = 0;
 pub const TCSADRAIN: ::c_int = 1;
 pub const TCSAFLUSH: ::c_int = 2;