SPIFFS: Fixed linux build and travis build

mkspiffs utility explicitly use gcc to compile

extras/spiffs/mkspiffs/Makefile

@@ -11,6 +11,10 @@ $(error $(ERROR_MSG))
 endif
 endif
 
+# explicitly use gcc as in xtensa build environment it might be set to
+# cross compiler
+CC = gcc
+
 SOURCES := spiffs_hydrogen.c
 SOURCES += spiffs_cache.c
 SOURCES += spiffs_gc.c

extras/spiffs/mkspiffs/mkspiffs.c

@@ -26,7 +26,7 @@
 #include <stdio.h>
 #include <sys/types.h>
 #include <sys/dir.h>
-#include <sys/dirent.h>
+#include <dirent.h>
 #include <stdint.h>
 #include <stdbool.h>
 #include <stdlib.h>