Hide ruby native extension symbols on FreeBSD

They probably should be hidden all every ELF platforms, but for
now only do it on Darwin, Linux, and FreeBSD.

This prevents some load-time errors that have been seen in
GitLab's gitaly server, and in a few other places.

ruby/ext/google/protobuf_c/extconf.rb

@@ -6,7 +6,7 @@
 
 dir_config(ext_name)
 
-if RUBY_PLATFORM =~ /darwin/ || RUBY_PLATFORM =~ /linux/
+if RUBY_PLATFORM =~ /darwin/ || RUBY_PLATFORM =~ /linux/ || RUBY_PLATFORM =~ /freebsd/
   $CFLAGS += " -std=gnu99 -O3 -DNDEBUG -fvisibility=hidden -Wall -Wsign-compare -Wno-declaration-after-statement"
 else
   $CFLAGS += " -std=gnu99 -O3 -DNDEBUG"