package/mosquitto: make broker optional

The mosquitto package provides both the MQTT client library and
a broker, and the latter may be not needed (when connecting to
a remote broker). It should be therefore possible to not install and
start it on the target

Also remove the dependency on BR2_TOOLCHAIN_HAS_SYNC_4, as it does not seem
to be needed.  Verified with:

 * br-m68k-68040-full.config  [OK]
 * br-sparc-uclibc.config     [OK]

The original issue adding the dependency in commit 874d0784
(package/mosquito: needs sync_4) unfortunately refers to autobuilder results
that are no longer available.

Signed-off-by: Titouan Christophe <titouan.christophe@railnova.eu>
[Peter: extend commit message, fix comment line, remove indentation in .mk]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>