.gitignore: Exclude the GKI generated defconfigs

Exclude the files generated by GKI scripts, such as:
  arch/*/configs/vendor/*-gki_defconfig
  arch/*/configs/vendor/*-qgki-debug_defconfig
  arch/*/configs/vendor/*-qgki_defconfig

These defconfigs are auto-generated at build time,
and should not be modified by the developers.

Change-Id: Ia965c23e2358b9ab0479b7415e2a60af5b892cd0
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>