Haiku: change argument types for readv() and writev()

This crate's definition was recently updated to conform to the actual type
definition in the header files. However, this deviates from the standard,
and thus breaks building - amongst others - libstd. This reverts to the
original definition. Parallel to this, there is an effort to fix the
definition on Haiku. In the interim, the disparity between the definitions
should not cause any issues for users that pass valid vector sizes.

Author: nielx

src/unix/haiku/mod.rs

@@ -874,6 +874,7 @@ pub const LOCK_UN: ::c_int = 0x08;
 
 pub const SIGSTKSZ: ::size_t = 16384;
 
+pub const IOV_MAX: ::c_int = 1024;
 pub const PATH_MAX: ::c_int = 1024;
 
 pub const SA_NOCLDSTOP: ::c_int = 0x01;
@@ -1502,12 +1503,12 @@ extern "C" {
     pub fn writev(
         fd: ::c_int,
         iov: *const ::iovec,
-        count: ::size_t,
+        count: ::c_int,
     ) -> ::ssize_t;
     pub fn readv(
         fd: ::c_int,
         iov: *const ::iovec,
-        count: ::size_t,
+        count: ::c_int,
     ) -> ::ssize_t;
 
     pub fn sendmsg(