Fix warning when LED_START_FLASHES is 0

flash_led() would be declared but not defined, which causes a warning
for static functions.

optiboot/bootloaders/optiboot/optiboot.c

@@ -359,7 +359,9 @@ void __attribute__((noinline)) verifySpace();
 void __attribute__((noinline)) watchdogConfig(uint8_t x);
 
 static inline void getNch(uint8_t);
+#if LED_START_FLASHES > 0
 static inline void flash_led(uint8_t);
+#endif
 static inline void watchdogReset();
 static inline void writebuffer(int8_t memtype, uint8_t *mybuff,
 			       uint16_t address, pagelen_t len);