lavc/ppc/h264dsp: Fix function prototype after bc26fe89.

Silences the following gcc warnings:
src/libavcodec/ppc/h264dsp.c: In function 'ff_h264dsp_init_ppc':
src/libavcodec/ppc/h264dsp.c:809:40: warning: assignment from incompatible pointer type
src/libavcodec/ppc/h264dsp.c:810:40: warning: assignment from incompatible pointer type
src/libavcodec/ppc/h264dsp.c:811:40: warning: assignment from incompatible pointer type
src/libavcodec/ppc/h264dsp.c:812:40: warning: assignment from incompatible pointer type
