gitignore: Ignore all build directories

Change-Id: I09d9323467cb90431f6ef3974a080d31292432e9
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/.gitignore b/.gitignore
index 378eac2..a0155cb 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1 @@
-build
+/build*/