Fix compilation of set_spec.c for previous Ruby versions

Set was moved to C only in Ruby 3.5, so it does not compile on Ruby 3.4
and earlier.

Author: peterzhu2118

spec/ruby/optional/capi/ext/set_spec.c

@@ -1,6 +1,7 @@
 #include "ruby.h"
 #include "rubyspec.h"
 
+#ifdef RUBY_VERSION_IS_3_5
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -60,3 +61,4 @@ void Init_set_spec(void) {
 }
 #endif
 
+#endif