net/mosquitto: Add PKG_LICENSE information

Signed-off-by: Karl Palsson <karlp@remake.is>

net/mosquitto/Makefile

 PKG_NAME:=mosquitto
 PKG_VERSION:=1.3.4
 PKG_RELEASE:=1
+PKG_LICENSE:=BSD-3-Clause
+PKG_LICENSE_FILE:=LICENSE.txt
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.gz
 PKG_SOURCE_URL:=http://mosquitto.org/files/source/