Fix stm32f4_cc311 example and add it to CI

The build has been broken because MQTT has been disabled during some refactoring
but never caught because this example was never built on CI.

PUBLISHED_FROM=b67051ff948f1ac89448fc707f41cdf5d48cdaaf

examples/STM32F4_CC3100/Makefile.build

+# -*- mode: makefile -*-
 .PHONY: all clean
 
 CC3100_DRV_SRCS = driver.c device.c nonos.c socket.c netapp.c wlan.c
@@ -60,9 +61,10 @@ INCDIRS = $(addprefix -I,$(IPATH))
 CFLAGS = ${FP_FLAGS} ${ARCH_FLAGS} ${INCDIRS}\
 		-D${BOARD} -DEXT_LIB_REGISTERED_GENERAL_EVENTS \
 		-DSL_FULL\
-		-DCS_PLATFORM=6\
-   		-D__STM32F407xx_H\
+		-DCS_PLATFORM=6 \
+		-D__STM32F407xx_H \
 		-DMG_SIMPLELINK_NO_OSI \
+		-DMG_ENABLE_MQTT \
 		-g -fno-common -ffunction-sections -fdata-sections \
 
 ELF = ${BUILD_DIR}/${PROJECT}.elf