Add ftruncate declaration to newlib unistd.h.
Add ftruncate declaration to newlib unistd.h.

While newlib itself doesn't implement this function,
the nosys library that we ship does, and the headers
should match this.   This also allows nacl_io to implement
ftruncate.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3363
R=mseaborn@chromium.org

Review URL: https://codereview.chromium.org/20883002

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@11871 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client