add UIO_MAXIOV macro in sys/uio.h

while there's no POSIX namespace provision for UIO_* in uio.h, this
exact macro name is reserved in XBD 2.2.2. apparently some
glibc-centric software expects it to exist, so let's provide it.

include/sys/uio.h

@@ -21,6 +21,8 @@ extern "C" {
 
 #include <bits/alltypes.h>
 
+#define UIO_MAXIOV 1024
+
 ssize_t readv (int, const struct iovec *, int);
 ssize_t writev (int, const struct iovec *, int);