Mk/Uses/metaport.mk: force LICENSE_COMB=single (+)

metaports should not have any license, so it was forced LICENSE to NA.
However, some metaports may inherit multi-licensing settings from master

which leads to failed validation check:

License not correctly defined: single license in LICENSE,

but LICENSE_COMB is set to dual (requires more than one)

Force paired variable LICENSE_COMB to 'single' to keep validation checks in consistent state

Discussed with: bapt
Approved by: portmgr (bapt)