androiddeployqt: Don't detect directories as executable files

When looking for an executable on the PATH we would mistakenly
report directories with the same filename (like "ant") as
the correct executable.

Task-number: QTBUG-33907
Change-Id: I7122e6f8da7a3027666e9dfc3040a17c2876e981
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>