fix: Fix a warning related to an unused argument

In the weak loader_port_debug_print() function, the string argument was
not explicitly marked as unused and caused unnecessary warnings.

Closes https://github.com/espressif/esp-serial-flasher/issues/81

src/protocol_common.c

@@ -297,5 +297,5 @@ esp_loader_error_t loader_spi_parameters(uint32_t total_size)
 
 __attribute__ ((weak)) void loader_port_debug_print(const char *str)
 {
-
+    (void) str;
 }