package/mosquito: needs sync_4

Fixes:
    http://autobuild.buildroot.org/results/2bc/2bc84ba2d1167018e2d48e5183ead22b6425dcf5/
    http://autobuild.buildroot.org/results/445/445f377ae70397b5f675f541977900e8986b79a4/
    http://autobuild.buildroot.org/results/57e/57e6984427f8c5d906a93884cc461b8f93cf5ce0/


    ...

[Peter: also add dependency to mosquitto comment]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>