Add non-gemimy/mkmf.rbs

Author: sue445

_gem/sig/non-gemify/mkmf.rbs

@@ -0,0 +1,5 @@
+module Kernel
+  def find_executable: (String bin, ?String path) -> String?
+
+  def create_makefile: (String target, String? srcprefix) -> void
+end