Add libgpg-error6-0 to the GCRYPT_ERROR_LIBRARY search list.

(Does FIND_LIBRARY support wildcards?)

Change-Id: I240e93070091e445446478b082ad284267c977a6
Reviewed-on: https://code.wireshark.org/review/3272
Reviewed-by: Gerald Combs <gerald@wireshark.org>