Fix broken UI when linking with devel/ncurses

Ports r438165 fixed building cmus with devel/ncurses installed, but
stripped NCURSES_CFLAGS too aggressivly. The build picked up base's
curses.h over the correct headers from devel/ncurses.

PR: 218548
Reported by: Oleg Gushchenkov <gor@clogic.com.ua>
Reviewed by: Oleg Gushchenkov <gor@clogic.com.ua>
Approved by: mat (mentor)
Differential Revision: https://reviews.freebsd.org/D10355