Restore erroneously removed .gitignore entry for "build" directory

It was removed in "delete autotools .gitignore files", but the build
directory is created by scons.

[Skip CI]

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
diff --git a/.gitignore b/.gitignore
index 52e4e61..a8917c2 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,3 @@
 *.pyc
 *.pyo
+build