groonga-httpd: fix wrong directory path
groonga-httpd: fix wrong directory path

Note that mruby-build is used to generate files which are required to
build mruby. It should not used from other vendor/*.

See http://www.clear-code.com/blog/2016/10/13.html

cosmo0920/groonga,komainu8/groonga,kenhys/groonga,naoa/groonga,komainu8/groonga,cosmo0920/groonga,cosmo0920/groonga,groonga/groonga,naoa/groonga,naoa/groonga,naoa/groonga,cosmo0920/groonga,komainu8/groonga,naoa/groonga,groonga/groonga,kenhys/groonga,cosmo0920/groonga,cosmo0920/groonga,kenhys/groonga,groonga/groonga,komainu8/groonga,groonga/groonga,komainu8/groonga,komainu8/groonga,naoa/groonga,naoa/groonga,cosmo0920/groonga,kenhys/groonga,komainu8/groonga,kenhys/groonga,kenhys/groonga,groonga/groonga,komainu8/groonga,groonga/groonga,groonga/groonga,kenhys/groonga,groonga/groonga,naoa/groonga,kenhys/groonga,cosmo0920/groonga