audio/mixertui: fix build on GCC architectures

Fix linking:
cc -L/usr/local/lib -lsysctlmibinfo2 -lmenu -lncurses mixertui.o -o mixertui
/usr/bin/ld: undefined reference to symbol `stdscr' (try adding -ltinfo)
/usr/local/lib/libtinfo.so: could not read symbols: Bad value