Refer to gtest with namespacing

It was not working for me otherwise.
Perhaps there's a better way.

cmake/QtProtobufInternalHelpers.cmake

@@ -57,8 +57,8 @@ function(qt_protobuf_internal_add_test)
         ${EXTRA_OPTIONS}
         PROTO_INCLUDES ${add_test_target_PROTO_INCLUDES})
 
-    target_link_libraries(${add_test_target_TARGET} PUBLIC gtest_main
-                                                           gtest
+    target_link_libraries(${add_test_target_TARGET} PUBLIC GTest::gtest_main
+                                                           GTest::gtest
                                                            ${QT_PROTOBUF_NAMESPACE}::Protobuf
                                                            ${QT_PROTOBUF_NAMESPACE}::Grpc
                                                            ${QT_VERSIONED_PREFIX}::Core

tests/test_qprotobuf_serializer_plugin/CMakeLists.txt

@@ -9,8 +9,8 @@ file(GLOB SOURCES
     serializationplugintest.cpp)
 
 add_executable(${TARGET} ${SOURCES})
-target_link_libraries(${TARGET} PRIVATE gtest_main
-                                        gtest
+target_link_libraries(${TARGET} PRIVATE GTest::gtest_main
+                                        GTest::gtest
                                         ${QT_PROTOBUF_NAMESPACE}::Protobuf
                                         ${QT_PROTOBUF_NAMESPACE}::Grpc
                                         Qt6::Core