Add GH_SUBDIR, automatically moves a secondary distfile to the right
place inside ${WRKSRC}.

Before:

GH_TUPLE= Regaddi:Chart.js:f13f99b:chart_js \

		FVANCOP:ChartNew.js:77e7f87:chartnew_js

post-extract:
@${RMDIR} ${WRKSRC}/database ${WRKSRC}/3rd/Chart.js
@${MV} ${WRKSRC_database} ${WRKSRC}/database
@${MV} ${WRKSRC_chart_js} ${WRKSRC}/3rd/Chart.js

After:

GH_TUPLE= Regaddi:Chart.js:f13f99b:chart_js/3rd/Chart.js \

		FVANCOP:ChartNew.js:77e7f87:chartnew_js/3rd/ChartNew.js

Sponsored by: Absolight
Differential Revision: https://reviews.freebsd.org/D7583