uclibc: enable UCLIBC_HAS_GNU_GETSUBOPT in all configs

This fixes build for applications that use getsubopt unconditionally.
Code addition to getopt is negligible.

Fixes:
  http://autobuild.buildroot.net/results/90c318e3a9ef5e626d0a80e36695d34eda21f97e/
  http://autobuild.buildroot.net/results/cd8e9d9a5f454c321071c21192958aeee9c39e73/



Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>