svq1.h where this function is used. Fixes the warning:
libavcodec/svq3.c:862: warning: implicit declaration of function ‘ff_svq1_packet_checksum’


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@15737 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b