a52dec, amrwbec: fix parse function declaration mismatch
Fixes MSVC compiler warning:
amrwbdec.c(99): warning C4133: '=': incompatible types
- from 'gboolean (__cdecl *)(GstAudioDecoder *,GstAdapter *,gint *,gint *)' to
'GstFlowReturn (__cdecl *)(GstAudioDecoder *,GstAdapter *,gint *,gint *)'
gsta52dec.c(156): warning C4133: '=': incompatible types
- from 'gboolean (__cdecl *)(GstAudioDecoder *,GstAdapter *,gint *,gint *)' to
'GstFlowReturn (__cdecl *)(GstAudioDecoder *,GstAdapter *,gint *,gint *)'