sig: declare CrossRubyProduct#dump_ast_path

Add the missing RBS declaration for dump_ast_path so steep accepts the new crossruby configure wiring.

Author: ledsun

sig/ruby_wasm/build.rbs

@@ -218,6 +218,7 @@ module RubyWasm
     def extinit_obj: -> String
     def extinit_c_erb: -> String
     def baseruby_path: -> String
+    def dump_ast_path: -> String
     def configure_args: (String build_triple, Toolchain toolchain) -> Array[String]
     def rbconfig_rb: -> String?
   end