newlib: Add declaration of readdir_r
newlib: Add declaration of readdir_r

This function is implemented in newlib (libcrt_common.a)
but was not declared in our sys/dirent.h header.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3988
TEST=newlib toolchain trybots
R=mseaborn@chromium.org

Review URL: https://codereview.chromium.org/686523004

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@14005 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

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