.gitignore: make junk visible for clean

I think flow folders were created in OpenROAD in the distant past?

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>

Author: oharboe

.gitignore

@@ -29,7 +29,6 @@ build
 build_linux
 build_mac
 test/results
-flow
 
 docs/main
 README2.md