Modify includes of config.h so that out-of-tree builds, i.e. CMake
don't pick up the in-tree copy.

Change-Id: I7ec473876cdba1a025c52362d7f6adc62d24ce71
Reviewed-on: https://code.wireshark.org/review/3798
Petri-Dish: Graham Bloice <graham.bloice@trihedral.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Roland Knall <rknall@gmail.com>
Reviewed-by: Graham Bloice <graham.bloice@trihedral.com>