Demo fail to compile when CONFIG_DEMO_CALCULATOR is disabled

[Eason0729]

When building the example binary with CONFIG_DEMO_CALCULATOR disabled in the kconfig, the compilation fails because the DEMO_WIN_X, DEMO_WIN_Y and get_demo_window_height are placed after the calc_depends in tests/example.c.

These symbols are used by all demo window drawing functions (clock, vector, nyan cat, theme, etc.). This causes compile error for other demos when calculator is disabled.