Link grpctest with libgrpc_unsecure explicitly (#4474)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index a4cccae..1537d83 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -235,7 +235,7 @@
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-unused-parameter -Wno-shadow")
   endif()
   add_executable(grpctest ${FlatBuffers_GRPCTest_SRCS})
-  target_link_libraries(grpctest grpc++_unsecure pthread dl)
+  target_link_libraries(grpctest grpc++_unsecure grpc_unsecure pthread dl)
 endif()
 
 if(FLATBUFFERS_INSTALL)