Add support for OpenBSD. Closes #315 (#326)

Author: vtamara

Diff for: ext/pg_query/ext_symbols_openbsd.sym

@@ -0,0 +1 @@
+Init_pg_query

Diff for: ext/pg_query/ext_symbols_openbsd_with_ruby_abi_version.sym

@@ -0,0 +1,2 @@
+Init_pg_query
+ruby_abi_version

Diff for: ext/pg_query/extconf.rb

@@ -35,6 +35,7 @@ def export_ruby_abi_version
 def ext_symbols_filename
   name = 'ext_symbols'
   name += '_freebsd' if RUBY_PLATFORM =~ /freebsd/
+  name += '_openbsd' if RUBY_PLATFORM =~ /openbsd/
   name += '_with_ruby_abi_version' if export_ruby_abi_version
   "#{name}.sym"
 end