windows: fix build error on MinGW (#1003)
windows: fix build error on MinGW (#1003)

It fixes the following error:

In function 'parser_yylex',
    inlined from 'yylex' at
/work/groonga/groonga.clean/vendor/mruby-source/mrbgems/mruby-compiler/core/parse.y:5979:10,
    inlined from 'yyparse' at
/work/groonga/groonga.clean/vendor/mruby-build/host/mrbgems/mruby-compiler/core/y.tab.c:5442:16:
/work/groonga/groonga.clean/vendor/mruby-source/mrbgems/mruby-compiler/core/parse.y:5012:11:
warning: 'strncat' output truncated before terminating nul copying 1
byte from a string of the same length [-Wstringop-truncation]
  CC       mruby-env/src/env.lo
  CC       mruby-file-stat/src/file-stat.lo
../../../groonga-9.0.4/vendor/mruby/mruby-file-stat/src/file-stat.c:11:10:
fatal error: config.h: No such file or directory
   11 | #include "config.h"
      |          ^~~~~~~~~~
komainu8/groonga,komainu8/groonga,groonga/groonga,komainu8/groonga,komainu8/groonga,kenhys/groonga,naoa/groonga,naoa/groonga,groonga/groonga,naoa/groonga,komainu8/groonga,naoa/groonga,naoa/groonga,kenhys/groonga,kenhys/groonga,komainu8/groonga,kenhys/groonga,kenhys/groonga,naoa/groonga,groonga/groonga,groonga/groonga,naoa/groonga,komainu8/groonga,groonga/groonga,kenhys/groonga,kenhys/groonga,groonga/groonga,naoa/groonga,groonga/groonga,groonga/groonga,komainu8/groonga,kenhys/groonga