[fix] missing stlink_fwrite_flash decl

TODO

@@ -1,4 +1,7 @@
 . flash writing is not working from GDB
+add a standalone application, flash/
+
+. add a startup.S based example
 
 . remove libsg dependency using:
 https://github.com/afaerber/stlink/tree/master/libstlink

src/stlink-common.h

@@ -211,6 +211,7 @@ extern "C" {
     // unprocessed
     int stlink_erase_flash_mass(stlink_t* sl);
     int stlink_write_flash(stlink_t* sl, stm32_addr_t address, uint8_t* data, unsigned length);
+    int stlink_fwrite_flash(stlink_t *sl, const char* path, stm32_addr_t addr);
     
     // PUBLIC
     uint16_t stlink_chip_id(stlink_t *sl);