Add include guard to SkFontConfigInterface.h

BUG=skia:7137

Change-Id: I29536a21211eae8b075d43984f3677f64ff9f481
Reviewed-on: https://skia-review.googlesource.com/57820
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>

src/ports/SkFontConfigInterface_direct.h

@@ -6,6 +6,8 @@
  */
 
 /* migrated from chrome/src/skia/ext/SkFontHost_fontconfig_direct.cpp */
+#ifndef SKFONTCONFIGINTERFACE_DIRECT_H_
+#define SKFONTCONFIGINTERFACE_DIRECT_H_
 
 #include "SkFontConfigInterface.h"
 
@@ -33,3 +35,5 @@ private:
                          const SkString& family);
     typedef SkFontConfigInterface INHERITED;
 };
+
+#endif