This fixes a bug when mosquitto is crosscompiled in LEDE on OS X.
UNAME is explicitly executed on the host, when we want it to be treated
as a regular linux build.

This patch passes the proper UNAME=Linux variable to the mosquitto
make file in order to respect linux as cross-compiler.

Signed-off-by: Thomas Huehn <thomas@net.t-labs.tu-berlin.de>
Reviewed-by: Karl Palsson <karlp@tweak.net.au>