spiffs: fix warning when building at release optimization level

Closes https://github.com/espressif/esp-idf/issues/4144

components/spiffs/CMakeLists.txt

@@ -10,3 +10,4 @@ idf_component_register(SRCS "esp_spiffs.c"
                     REQUIRES spi_flash
                     PRIV_REQUIRES bootloader_support)
 
+set_source_files_properties(spiffs/src/spiffs_nucleus.c PROPERTIES COMPILE_FLAGS -Wno-stringop-truncation)