gitignore /build*/ folders

Useful when making multiple out of source builds from the same source.
Also made it ignore only build folders from the root, in case there's
any "build*" subfolder in the tree.

Change-Id: I0140bb3aed73d2e88a6f1165f2b4c80fb99a8cf8
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/51141
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
diff --git a/.gitignore b/.gitignore
index 56d35271..31c9126 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,7 +6,7 @@
 .vscode/*
 !.vscode/tasks.json
 .idea
-build
+/build*/
 buildtools
 cmake-build-*/
 coverage.summary