Add CMake ALIAS targets

We follow the findProtobuf.cmake naming convention
to ease the use of protobuf as cmake subproject
diff --git a/cmake/libprotobuf.cmake b/cmake/libprotobuf.cmake
index e07340d..65d05c1 100644
--- a/cmake/libprotobuf.cmake
+++ b/cmake/libprotobuf.cmake
@@ -128,3 +128,4 @@
     VERSION ${protobuf_VERSION}
     OUTPUT_NAME ${LIB_PREFIX}protobuf
     DEBUG_POSTFIX "${protobuf_DEBUG_POSTFIX}")
+add_library(protobuf::libprotobuf ALIAS libprotobuf)