libc: Fix _obstack_newchunk public symbol vis

975bca165c3e10e74c05c0384fd58f45a7025a3c avoided relocations to
_obstack_newchunk used by obstack_vprintf but missed to enable the
corresponding public symbol which is used by the obstack macros.

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>