FindProtobuf: Set Protobuf_FOUND in addition to PROTOBUF_FOUND

All other modules use their module name (e.g. XxX for FindXxX.cmake) in
find_package_handle_standard_args. Protobuf used all-caps, which triggers
a bug when we try to find Protobuf with the CMakeFindDependencyMacro.cmake
macro, which only checks for the mixed-case _FOUND.

Modules/FindProtobuf.cmake

@@ -305,7 +305,7 @@ mark_as_advanced(PROTOBUF_PROTOC_EXECUTABLE)
 
 
 include(${CMAKE_CURRENT_LIST_DIR}/FindPackageHandleStandardArgs.cmake)
-FIND_PACKAGE_HANDLE_STANDARD_ARGS(PROTOBUF DEFAULT_MSG
+FIND_PACKAGE_HANDLE_STANDARD_ARGS(Protobuf DEFAULT_MSG
     PROTOBUF_LIBRARY PROTOBUF_INCLUDE_DIR)
 
 if(PROTOBUF_FOUND)