GD32: add CAN driver for GD32F4xx

Author: Dimatcher

bsp/gd32/arm/libraries/gd32_drivers/SConscript

@@ -56,6 +56,10 @@ if GetDepend('RT_USING_SDIO'):
 if GetDepend('RT_USING_PWM'):
     src += ['drv_pwm.c']
 
+# add can drivers.
+if GetDepend('RT_USING_CAN'):
+    src += ['drv_can.c']
+
 # add sdram drivers.
 if GetDepend('BSP_USING_SDRAM'):
     src += ['drv_sdram.c']