Ignore out-of-tree build directories

The first .gitignore entry was "./build/*". A leading ./ is not a valid
gitignore pattern, so it matched nothing and build/ turned up as
untracked in every git status. Use build*/, which also covers the
instrumented trees scripts/coverage.py and -DAKGL_COVERAGE=ON create.

Nothing under a build* path is tracked, so this hides no existing file.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitignore

@@ -1,4 +1,7 @@
-./build/*
+# A leading ./ is not a valid gitignore pattern, so "./build/*" matched nothing
+# and every out-of-tree build tree showed up as untracked. This also covers the
+# instrumented trees scripts/coverage.py and the AKGL_COVERAGE option create.
+build*/
 .aider*
 *~