Avoid calling rb_ext_ractor_safe on 2.x Rubies

Can you tell it's Friday?

ext/datadog_profiling_native_extension/profiling.c

@@ -43,7 +43,9 @@ static VALUE _native_safe_object_info(DDTRACE_UNUSED VALUE _self, VALUE obj);
 void DDTRACE_EXPORT Init_datadog_profiling_native_extension(void) {
   // The profiler still has a lot of limitations around being used in Ractors BUT for now we're choosing to take care of those
   // on our side, rather than asking Ruby to block calling our APIs from Ractors.
-  rb_ext_ractor_safe(true);
+  #ifdef HAVE_RB_EXT_RACTOR_SAFE
+    rb_ext_ractor_safe(true);
+  #endif
 
   VALUE datadog_module = rb_define_module("Datadog");
   VALUE profiling_module = rb_define_module_under(datadog_module, "Profiling");