Add UIO_MAXIOV constant to uclibc

!1880 added UIO_MAXIOV to libc, but did not add it to uclibc. This
causes build breaks for std against uclibc.

src/unix/uclibc/mod.rs

@@ -932,6 +932,8 @@ pub const SS_DISABLE: ::c_int = 2;
 
 pub const PATH_MAX: ::c_int = 4096;
 
+pub const UIO_MAXIOV: ::c_int = 1024;
+
 pub const FD_SETSIZE: usize = 1024;
 
 pub const EPOLLIN: ::c_int = 0x1;