[bsp][stm32f107] fix compile error. add missing macro RT_USING_LIBC

bsp/stm32f107/rtconfig.h

@@ -163,4 +163,5 @@
 // <bool name="RT_USING_BSP_CMSIS" description="Using CMSIS in BSP" default="true" />
 // #define RT_USING_BSP_CMSIS
 
+#define RT_USING_LIBC
 #endif