textproc/asciidoc: explicit the dependencies on libxml2 and libxslt

With newer pkg we plan to dissociate the semantic of LIB_DEPENDS and
RUN_DEPENDS, asciidoc does not actually depend on the library part of
both libxml2 and libxslt but on the binaries xmllint and xsltproc.

USE_GNOME= libxml2 libxslt will result with the runtime dependencies
expressed a LIB_DEPENDS which make the packages broken with some
upcoming changes in the ports tree and newer pkg.