Add prototype for systick_attach_callback (thanks to @ag123)

Author: rogerclarkmelbourne

STM32F1/system/libmaple/include/libmaple/systick.h

@@ -108,6 +108,12 @@ static inline uint32 systick_check_underflow(void) {
     return SYSTICK_BASE->CSR & SYSTICK_CSR_COUNTFLAG;
 }
 
+/**
+ * @brief prototype for systick_attach_callback
+ *
+ */
+extern void systick_attach_callback(void (*callback)(void));
+
 #ifdef __cplusplus
 } // extern "C"
 #endif