Requires sdl2 rather than sdl for pkgconfig file

[fundawang]

In SDL/CMakeLists.txt, sdl2 was detected but sdl was written in output .pc file. It should be sdl2 instead.

[fundawang]

Or, it should be sdl 1.x rather than sdl 2.x when detecting?

[gdraheim]

The sdl-example is based on SDL2, so that's a better thing to do.

The interesting point here is that the same problem exists in the Makefile.am code. I am not sure if something has changed when SDL3 was released in 2022, or that the problem went undetected for about ten years since SDL2 was released in 2013.