Revert "ANDROID: add gki_module headers to .gitignore file"

This reverts commit 2f0acef0.

The headers are no longer generated since commit bf7b3766 ("Revert
"ANDROID: GKI: Add script to generate symbol protection headers""), so
these are no longer needed in gitignore. Plus they cause merge conflicts
with upstream changes.

Change-Id: Iecae47593f619c9c99b65f0837828a023c358d8a
Signed-off-by: Carlos Llamas <cmllamas@google.com>