stm32l0: add exti to makefile

Tested with a Lora radio application that used three GPIO lines to send
different interrupts from the radio module to the L052. Tested with a
Jeenode Zero Rev1

lib/stm32/l0/Makefile

@@ -42,6 +42,7 @@ OBJS		+= timer_common_all.o
 OBJS            += gpio_common_all.o gpio_common_f0234.o rcc_common_all.o
 OBJS		+= adc_common_v2.o
 OBJS		+= crs_common_all.o
+OBJS		+= exti_common_all.o
 
 OBJS            += usb.o usb_control.o usb_standard.o
 OBJS            += st_usbfs_core.o st_usbfs_v2.o