Update pcg_basic.c function definition to match header

[ianfhunter]

If compiled with -Wstrict-prototypes using emcc (the emscripten compiler), it successfully catches that this function header is empty (though it is correctly 'void' in the associated header)

error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
   67 | uint32_t pcg32_random() { return pcg32_random_r(&pcg32_global); }
      |                      ^
      |                       void