fix libcurses dependency on OpenBSD

$ make test
PERL_DL_NONLAZY=1 /usr/bin/perl "-MExtUtils::Command::MM" "-e" "test_harness(0, 'blib/lib', 'blib/arch')" t/*.t
t/1.t .. /usr/bin/perl:/usr/lib/libedit.so.4.1: undefined symbol 'tgetnum'
/usr/bin/perl:/usr/lib/libedit.so.4.1: undefined symbol 'tgoto'
/usr/bin/perl:/usr/lib/libedit.so.4.1: undefined symbol 'tgetflag'
/usr/bin/perl:/usr/lib/libedit.so.4.1: undefined symbol 'tputs'
/usr/bin/perl:/usr/lib/libedit.so.4.1: undefined symbol 'tgetent'
/usr/bin/perl:/usr/lib/libedit.so.4.1: undefined symbol 'tgetstr'
t/1.t .. ok
All tests successful.
Files=1, Tests=2,  0 wallclock secs ( 0.02 usr  0.02 sys +  0.01 cusr  0.02 csys =  0.07 CPU)
Result: PASS
$

The attached patch adds the libcurses dependency on OpenBSD (checking for openbsd in $^O). prevents the undefined symbol warnings and lets Term::ReadLine::EditLine install.