Merge pull request #2489 from yang-g/header_guard

set header guard

include/grpc++/fixed_size_thread_pool.h

@@ -31,8 +31,8 @@
  *
  */
 
-#ifndef GRPC_INTERNAL_CPP_SERVER_THREAD_POOL_H
-#define GRPC_INTERNAL_CPP_SERVER_THREAD_POOL_H
+#ifndef GRPCXX_FIXED_SIZE_THREAD_POOL_H
+#define GRPCXX_FIXED_SIZE_THREAD_POOL_H
 
 #include <grpc++/config.h>
 
@@ -64,4 +64,4 @@ class FixedSizeThreadPool GRPC_FINAL : public ThreadPoolInterface {
 
 }  // namespace grpc
 
-#endif  // GRPC_INTERNAL_CPP_SERVER_THREAD_POOL_H
+#endif  // GRPCXX_FIXED_SIZE_THREAD_POOL_H