Add CMake ALIAS targets

We follow the findProtobuf.cmake naming convention
to ease the use of protobuf as cmake subproject
diff --git a/cmake/libprotoc.cmake b/cmake/libprotoc.cmake
index a85dccd..5f6d078 100644
--- a/cmake/libprotoc.cmake
+++ b/cmake/libprotoc.cmake
@@ -228,3 +228,4 @@
     VERSION ${protobuf_VERSION}
     OUTPUT_NAME ${LIB_PREFIX}protoc
     DEBUG_POSTFIX "${protobuf_DEBUG_POSTFIX}")
+add_library(protobuf::libprotoc ALIAS libprotoc)