package/dropwatch: license is GPL-2.0+

Dropwatch links with readline, which is GPL-3.0+, so not compatible with
GPL-2.0.  When asked about this, upstream has clarified that the license
really is GPL-2.0+:

https://github.com/nhorman/dropwatch/issues/14



Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
[yann.morin.1998@free.fr: add a coomet as suggested by Baruc]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Baruch Siach <baruch@tkos.co.il>