onigmo: 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/dmbvc2013/powershell/work/source/storage/mroonga/vendor/groonga/vendor/onigmo

  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.
```

vendor/onigmo/Makefile.am

@@ -1,5 +1,6 @@
 EXTRA_DIST =					\
-	configure
+	configure				\
+	CMakeLists.txt
 
 CONFIGURE_DEPENDENCIES =			\
 	configure