gitignore:  drop build/ from gitignore again
gitignore:  drop build/ from gitignore again

As pointed out elsewhere, the build directory is decided by the user and
should thus be a user-specific gitignore configuration.

Signed-off-by: Peter Hutterer <b9c50af7afa7642f6f854434cda9ffe54ece8b85@who-t.net>

whot/libratbag,whot/libratbag,libratbag/libratbag,libratbag/libratbag,whot/libratbag,libratbag/libratbag