Fix _gRPC_PROTOBUF_WELLKNOWN_INCLUDE_DIR NOT-FOUND

cmake/protobuf.cmake

@@ -65,12 +65,11 @@ elseif(gRPC_PROTOBUF_PROVIDER STREQUAL "package")
     endif()
     if(TARGET protobuf::libprotoc)
       set(_gRPC_PROTOBUF_PROTOC_LIBRARIES protobuf::libprotoc)
-      # extract the include dir from target's properties
-      get_target_property(_gRPC_PROTOBUF_WELLKNOWN_INCLUDE_DIR protobuf::libprotoc INTERFACE_INCLUDE_DIRECTORIES)
     else()
       set(_gRPC_PROTOBUF_PROTOC_LIBRARIES ${PROTOBUF_PROTOC_LIBRARIES})
-      set(_gRPC_PROTOBUF_WELLKNOWN_INCLUDE_DIR ${PROTOBUF_INCLUDE_DIRS})
     endif()
+    # Well-known proto files are expected to be in the Protobuf include directory.
+    get_target_property(_gRPC_PROTOBUF_WELLKNOWN_INCLUDE_DIR _gRPC_PROTOBUF_LIBRARIES INTERFACE_INCLUDE_DIRECTORIES)
     if(TARGET protobuf::protoc)
       set(_gRPC_PROTOBUF_PROTOC protobuf::protoc)
       if(CMAKE_CROSSCOMPILING)