Explicitly declare function parameters for GCC 15 compatibility

In GCC 15, where C23 is the default, void foo() is equivalent to void foo(void). Therefore, functions must explicitly declare their parameters.

../src/process_pipeline.c: In function ‘mp_process_pipeline_capture’:
../src/process_pipeline.c:1426:38: error: passing argument 2 of ‘mp_pipeline_invoke’ from incompatible pointer type [-Wincompatible-pointer-types]
 1426 |         mp_pipeline_invoke(pipeline, capture, NULL, 0);
      |                                      ^~~~~~~
      |                                      |
      |                                      void (*)(void)
In file included from ../src/process_pipeline.c:6:
../src/pipeline.h:12:44: note: expected ‘MPPipelineCallback’ {aka ‘void (*)(struct _MPPipeline *, const void *)’} but argument is of type ‘void (*)(void)’
   12 |                         MPPipelineCallback callback,
      |                         ~~~~~~~~~~~~~~~~~~~^~~~~~~~