mapi/test: Change type to unsigned for offset

Quiets this warning:

../../master/src/mapi/glapi/tests/check_table.cpp:576:20: error: non-constant-expression cannot be narrowed from type 'unsigned int' to 'int' in initializer list [-Wc++11-narrowing]
   { "glColor3dv", _O(Color3dv) },
                   ^~~~~~~~~~~~

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6112>