Ignore build32 and build64 subdirectories.

Used for Conscrypt local builds:
https://github.com/google/conscrypt/blob/master/BUILDING.md

Change-Id: I7eaa225cdc3c79cb5ac4902d222d408169f7ded0
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/37724
Commit-Queue: Pete Bentley <prb@google.com>
Reviewed-by: Adam Langley <agl@google.com>

.gitignore

diff --git a/.gitignore b/.gitignore
index 366feb7..8ce23d1 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,6 @@
 build/
+build32/
+build64/
 ssl/test/runner/runner
 *.pyc
 *.swp