Fix Linux's CMSG_NXTHDR definition.

This is an error from PR rust-lang#1098.  The wrong definition coincidentally
works on Linux/glibc, but it fails on Linux/musl.

Author: asomers

src/unix/notbsd/mod.rs

@@ -999,7 +999,7 @@ f! {
         if cmsg.is_null() {
             return CMSG_FIRSTHDR(mhdr);
         };
-        let pad = mem::align_of::<cmsghdr>() - 1;
+        let pad = mem::align_of::<usize>() - 1;
         let next = cmsg as usize + (*cmsg).cmsg_len as usize + pad & !pad;
         let max = (*mhdr).msg_control as usize
             + (*mhdr).msg_controllen as usize;