[core] Remove rcore.h include from SDL

[ghost]

Changes

1. Very short fix to remove the rcore.h include from rcore_desktop_sdl.c (L52) so it matches the other submodules.
   References: #3420, #3429.

Environment

Changes successfully tested on Linux (Ubuntu 22.04 64-bit, gcc 11.2.0) with SDL2 (2.28.4).

Edits

1: added line marks.

[ghost]

@raysan5 After this change, the only other file using rcore.h is core.c.

Since rcore.h is just some defines and the CoreData struct, perhaps it would make sense to move it inside rcore.c?

[raysan5]

Since rcore.h is just some defines and the CoreData struct, perhaps it would make sense to move it inside rcore.c?

@ubkp thanks for the review! And, yeah, that's right, rcore.h could probably be moved back to rcore.c.