webkitgtk: select libgcrypt

Libgrcrypt is a direct dependency of WebKitGTK+, and as such it
should be selected.

Signed-off-by: Adrian Perez de Castro <aperez@igalia.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Thomas: add missing dependency on BR2_PACKAGE_LIBGPG_ERROR_ARCH_SUPPORTS.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>