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-lite.cmake b/cmake/libprotobuf-lite.cmake
index 423b6c1..2df6a29 100644
--- a/cmake/libprotobuf-lite.cmake
+++ b/cmake/libprotobuf-lite.cmake
@@ -65,3 +65,4 @@
     VERSION ${protobuf_VERSION}
     OUTPUT_NAME ${LIB_PREFIX}protobuf-lite
     DEBUG_POSTFIX "${protobuf_DEBUG_POSTFIX}")
+add_library(protobuf::libprotobuf-lite ALIAS libprotobuf-lite)