package/erlang: fix build issue with pcre package

If the pcre package is build before erlang, the erlang's build
system use pcre.h from pcre package instead of using pcre.h bundled
by Erlang.

Erlang use an old version of this file which is incompatible
with the upstream one.

http://autobuild.buildroot.net/results/cbd/cbd8b54eef535f19d7d400fd269af1b3571d6143/build-end.log



And many more.

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>