Fix undefined termcap symbols on OpenBSD.

E.g.:
/usr/lib/libreadline.so.4.0: undefined reference to `tgetnum'
/usr/lib/libreadline.so.4.0: undefined reference to `tgoto'
/usr/lib/libreadline.so.4.0: undefined reference to `tgetflag'
/usr/lib/libreadline.so.4.0: undefined reference to `tputs'
/usr/lib/libreadline.so.4.0: undefined reference to `tgetent'
/usr/lib/libreadline.so.4.0: undefined reference to `tgetstr'

Tested on linux too, works.