Define prototypes for ZSTD custom allocation functions.

[reshke]

fixes:

buffile.c:1039:1: warning: no previous prototype for ‘customAlloc’ [-Wmissing-prototypes]
 1039 | customAlloc(void *opaque, size_t size)
      | ^~~~~~~~~~~
buffile.c:1045:1: warning: no previous prototype for ‘customFree’ [-Wmissing-prototypes]
 1045 | customFree(void *opaque, void *address)
      | ^~~~~~~~~~

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