Add build subprojects to .gitignore

Add blueprint, kati, and soong subdirectories to .gitignore so nothing
gets committed there when new subprojects are added to the manifest.

Change-Id: I888b94a89c87d99c8d28e8abd07a830f9cd86702
diff --git a/.gitignore b/.gitignore
index c9b568f..f1f4a52 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,5 @@
 *.pyc
 *.swp
+blueprint/
+kati/
+soong/