imx8mm_evk: Include sys_proto.h header

Add sys_proto.h header file to fix the following sparse warning:

board/freescale/imx8mm_evk/imx8mm_evk.c:59:5: warning: no previous
prototype for ‘board_mmc_get_env_dev’ [-Wmissing-prototypes]

Signed-off-by: Alifer Moraes <alifer.wsdm@gmail.com>

board/freescale/imx8mm_evk/imx8mm_evk.c

@@ -8,6 +8,7 @@
 #include <netdev.h>
 
 #include <asm/arch/clock.h>
+#include <asm/arch/sys_proto.h>
 #include <asm/io.h>
 
 DECLARE_GLOBAL_DATA_PTR;