Ignore also instdir_for_build and workdir_for_build

Change-Id: I2e6a26cd1e54ad28ec27f2205262413db039a136
diff --git a/.gitignore b/.gitignore
index a227af3d..64ebb3c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -21,7 +21,9 @@

# where stuff are built
/instdir
/instdir_for_build
/workdir
/workdir_for_build

# where the external downloads maybe located
/external/tarballs