.gitignore: only ignore build directories

The "build*" pattern is ignoring any file that begins with
"build". Add a trailing "/" to limit the effect of the pattern
to directories.

This continues to ignore build directories, but allows adding files
whose names begin with build, which will be used later to create a
build.rst file for documenting west's build command.

Signed-off-by: Marti Bolivar <marti@foundries.io>