textproc/apertium: Change BUILD_DEPENDS from utf8cpp3 to utf8cpp

The build fix for utf8cpp 4.0.x is part of apertium 3.9.4.

Reference: https://github.com/apertium/apertium/commit/1211c1a2ed6c8b615bab1dc5b11dd340ece40368
Approved by: portmgr (blanket)