mrb: add EXTRA_DIST to suppress CMP0014 warning

following warning generates building mariadb bundled Mroonga nightly package:

```log
CMake Warning (dev) at
storage/mroonga/vendor/groonga/vendor/CMakeLists.txt:16
(add_subdirectory):
  The source directory

  C:/jw/workspace/dmb/powershell/work/source/storage/mroonga/vendor/groonga/vendor/mruby

  does not contain a CMakeLists.txt file.

  CMake does not support this case but it used to work
  accidentally and is
  being allowed for compatibility.

  Policy CMP0014 is not set: Input directories must have CMakeLists.txt. Run
  "cmake --help-policy CMP0014" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.
  This warning is for project developers.  Use -Wno-dev to suppress it.
```
Merge pull request #201 from cosmo0920/suppress-cmp0014

mrb: add EXTRA_DIST to suppress CMP0014 warning

Patch by cosmo0920. Thanks!!!