Make precache extern "C"

cores/esp8266/core_esp8266_features.cpp

@@ -28,7 +28,7 @@
  *  otherwise preloads flash at the given address.
  *  All preloads are word aligned.
  */
-void precache(void *f, uint32_t bytes) {
+extern "C" void precache(void *f, uint32_t bytes) {
   // Size of a cache page in words. We only need to read one word per
   // page (ie 1 word in 8) for this to work.
   #define CACHE_PAGE_SIZE (32/4)

cores/esp8266/core_esp8266_features.h

@@ -104,6 +104,6 @@ inline uint32_t esp_get_cycle_count() {
 #define PRECACHE_END(tag) \
     _precache_end_##tag:
 
-void precache(void *f, uint32_t bytes);
+extern "C" void precache(void *f, uint32_t bytes);
 
 #endif // CORE_ESP8266_FEATURES_H