mosquitto: Fix host UNAME detection

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>

net/mosquitto/Makefile

@@ -209,7 +209,7 @@ define Package/libmosquittopp/install
 endef
 
 # Applies to all...
-MAKE_FLAGS += WITH_DOCS=no
+MAKE_FLAGS += WITH_DOCS=no UNAME=Linux
 ifeq ($(BUILD_VARIANT),nossl)
 	MAKE_FLAGS += WITH_TLS=no WITH_WEBSOCKETS=no
 else