dovecot: fix static build

Add a patch removing reference to MODULE_SUFFIX when it is undefined.

Fixes:
http://autobuild.buildroot.net/results/c68/c6844bbffff1cd4f738a5fced011d28f73c90b16/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>