Fixed setting custom rbfiles in gemspec

tasks/mrbgem_spec.rake

@@ -63,9 +63,6 @@ module MRuby
           objfile(f.relative_path_from(@dir).to_s.pathmap("#{build_dir}/%X"))
         end
 
-        @generate_functions = !(@rbfiles.empty? && @objs.empty?)
-        @objs << objfile("#{build_dir}/gem_init") if @generate_functions
-
         @test_rbfiles = Dir.glob("#{dir}/test/**/*.rb")
         @test_objs = Dir.glob("#{dir}/test/*.{c,cpp,cxx,cc,m,asm,s,S}").map do |f|
           objfile(f.relative_path_from(dir).to_s.pathmap("#{build_dir}/%X"))
@@ -83,6 +80,9 @@ module MRuby
 
         instance_eval(&@initializer)
 
+        @generate_functions = !(@rbfiles.empty? && @objs.empty?)
+        @objs << objfile("#{build_dir}/gem_init") if @generate_functions
+
         if !name || !licenses || !authors
           fail "#{name || dir} required to set name, license(s) and author(s)"
         end