Windows: Add Qt Creator PATH update tip for missing SQL client dlls

This patch aims to provide an additional tip for users having trouble
with e.g. the MySQL plugin. After a successful build most users are
struggling with loading failure due the fact that the client libraries
folder is not contained in the PATH environment variable. This tip
helps them overcome that in a controlled manner with Qt Creator.

Change-Id: I2dc5c9c6d8d8976686d74c369b6e1683c479f35c
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>