[bsp][cv1800b] fix a implicit-function-declaration warning

Signed-off-by: LiuLingze <luiyanbing@foxmail.com>

libcpu/risc-v/t-head/c906/trap.c

@@ -28,6 +28,7 @@
 
 #ifdef RT_USING_SMART
 #include <lwp_arch.h>
+void rt_hw_backtrace(rt_uint32_t *ffp, rt_ubase_t sepc);
 #else
 #define rt_hw_backtrace(...) (0)
 #endif