gitignore /build-*

I like to create multiple build folders at the root for different types
of CMake builds:

build-clang
build_asan
etc.

Don't want these showing up in git status.

Change-Id: I03fe5a812f7090e6d6481a5b07bc8c95a33ac106
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/90601
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>

.gitignore

@@ -121,6 +121,7 @@ coverage.summary
 default.profraw
 lcov.info
 /cmake-build-*/
+/build-*/
 /testing
 
 ### Clang-Tidy files