[ruby/etc] Remove unnecessary declaration

Fix https://github.com/ruby/etc/pull/12

https://github.com/ruby/etc/commit/7cbf03d22d

ext/etc/extconf.rb

@@ -47,10 +47,7 @@ if !File.exist?("#{srcdir}/depend")
   %x[#{RbConfig.ruby} #{srcdir}/mkconstants.rb -o #{srcdir}/constdefs.h]
 end
 
-decl = [
-  "void rb_deprecate_constant(VALUE, const char *);",
-]
-have_func('rb_deprecate_constant(Qnil, "None")', [decl])
+have_func('rb_deprecate_constant(Qnil, "None")')
 
 $distcleanfiles << "constdefs.h"