[ruby/mmtk] Fix rustsrcs in extconf.rb

https://github.com/ruby/mmtk/commit/7e9aadd29e

gc/mmtk/extconf.rb

@@ -5,7 +5,7 @@ require_relative "../extconf_base"
 # Statically link `libmmtk_ruby.a`
 $LIBS << " $(MMTK_BUILD)/$(LIBMMTK_RUBY)"
 
-rustsrcs = Dir.glob("src/*.rs", base: __dir__).map {|s| "$(srcdir)/#{s}"}
+rustsrcs = Dir.glob("src/**/*.rs", base: __dir__).map {|s| "$(srcdir)/#{s}"}
 
 create_gc_makefile("mmtk") do |makefile|
   [