SDL audio callback doesn't work on Windows

[bunnylin]

The basic audio callback procedure works correctly on fpc/linux, but not on fpc/windows.

To fix, the definition of TSDL_AudioCallback in sdlaudio.inc:118 must be given the cdecl modifier.

(The TSDL_AudioFilter definition on line 138 probably needs cdecl too, though I didn't test that.)

[ev1313]

Fixed.