in SDL_rect.h: remove inclusion of SDL_pixels.h and SDL_rwops.h

[Sackzement]

I don't see SDL_rect.h using anything from the two headers. And files which include SDL_rect.h(which are SDL_surface.h, SDL_video.h and SDL_render.h) already include SDL_pixels.h and/or SDL_rwops.h, if they use them.

Building SDL with SDL_TESTS works.

Edit: Should have posted this issue under the ABI-breaking milestone. Is there a way for authors to change the milestone without reposting the issue?