defining __USE_XOPEN for MAX_IOV.

On old Linux, network 2.4.1.0 cannot be compiled:

    Preprocessing library network-2.4.1.0...
    ByteString.hsc: In function ‘main’:
    ByteString.hsc:204: error: ‘IOV_MAX’ undeclared (first use in this function)
    ByteString.hsc:204: error: (Each undeclared identifier is reported only once
    ByteString.hsc:204: error: for each function it appears in.)
    compiling dist/build/Network/Socket/ByteString_hsc_make.c failed (exit code 1)
    command was: /home/kazu/bin/gcc -c dist/build/Network/Socket/ByteString_hsc_make.c -o dist/build/Network/Socket/ByteString_hsc_make.o -fno-stack-protector -fno-stack-protector -D__GLASGOW_HASKELL__=700 -Dlinux_BUILD_OS -Dlinux_HOST_OS -Di386_BUILD_ARCH -Di386_HOST_ARCH -Iinclude -DCALLCONV=ccall -I/ghc7.0.4/lib/ghc-7.0.4/unix-2.4.2.0/include -I/ghc7.0.4/lib/ghc-7.0.4/bytestring-0.9.1.10/include -I/ghc7.0.4/lib/ghc-7.0.4/base-4.3.1.0/include -I/ghc7.0.4/lib/ghc-7.0.4/include -I/ghc7.0.4/lib/ghc-7.0.4/include -I/ghc7.0.4/lib/ghc-7.0.4/include/

This patch fixes this problem.