Fix compile warning in util/xfunc.h

Summary:
./util/xfunc.h:31:1: error: class 'Options' was previously declared as a struct [-Werror,-Wmismatched-tags]
class Options;
^

Test Plan:
make dbg -j32

util/xfunc.h

@@ -28,7 +28,7 @@ namespace rocksdb {
 #define XFUNC_TEST(condition, location, lfname, fname, ...)
 #else
 
-class Options;
+struct Options;
 void GetXFTestOptions(Options* options, int skip_policy);
 
 // This class provides the facility to run custom code to test a specific