Don't ignore openbsd (the stated reason doesn't apply to our case)

Author: jieyouxu

tests/ui/debuginfo/backtrace-line-tables-only.rs

@@ -9,7 +9,6 @@
 //@ run-pass
 //@ compile-flags: -Cstrip=none -Cdebuginfo=line-tables-only
 //@ ignore-android FIXME #17520
-//@ ignore-openbsd no support for libbacktrace without filename
 //@ ignore-fuchsia Backtraces not symbolized
 //@ needs-unwind
 //@ aux-build: line-tables-only-helper.rs