Introduce prototype-style function definitions for readv and writev.
Upstream, the old-style function definitions were removed in:

commit 42b7f5d48549b85386a9b28a1a90e66fd81ba273
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Mon Oct 20 15:49:08 2014 +0000

    Move readv and writev definitions to syscalls.list (bug 14138).

diff --git a/sysdeps/unix/sysv/linux/readv.c b/sysdeps/unix/sysv/linux/readv.c
index e8acd65ce377963a..e49d54ede39d45af 100644
--- a/sysdeps/unix/sysv/linux/readv.c
+++ b/sysdeps/unix/sysv/linux/readv.c
@@ -39,10 +39,7 @@ static ssize_t __atomic_readv_replacement (int, const struct iovec *,
 
 
 ssize_t
-__libc_readv (fd, vector, count)
-     int fd;
-     const struct iovec *vector;
-     int count;
+__libc_readv (int fd, const struct iovec *vector, int count)
 {
   ssize_t result;
 
diff --git a/sysdeps/unix/sysv/linux/writev.c b/sysdeps/unix/sysv/linux/writev.c
index 8b018ecdf6fbc323..847a1057aa443238 100644
--- a/sysdeps/unix/sysv/linux/writev.c
+++ b/sysdeps/unix/sysv/linux/writev.c
@@ -38,10 +38,7 @@ static ssize_t __atomic_writev_replacement (int, const struct iovec *,
 
 
 ssize_t
-__libc_writev (fd, vector, count)
-     int fd;
-     const struct iovec *vector;
-     int count;
+__libc_writev (int fd, const struct iovec *vector, int count)
 {
   ssize_t result;