collectd: fix musl builds (ipc module)

src/ipc.c uses shm_info.used_ids which musl only provides when
_GNU_SOURCE is defined.

The issue has been fixed upstream, but the fix is not in their
latest 5.5.0 release. Adding their patch as a temporary fix
to be removed on the next version bump.

https://github.com/collectd/collectd/issues/1147
https://github.com/collectd/collectd/commit/3e3848349b753d78a0b1d19648fb394866856bda

Fixes
http://autobuild.buildroot.net/results/e981c08f01d2916246a17b1dc112e8b3e8c628e2/



Signed-off-by: Alex Suykov <alex.suykov@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>