Fix compilation bug for gcc version 8.3.0 (Debian 8.3.0-6)

[yweber]

When compiling on Debian Buster ( gcc version 8.3.0 (Debian 8.3.0-6) ), ld fails with :

/usr/bin/ld: /tmp/amv-ffmpeg/libavcodec/libavcodec.a(motion_est.o): in function ff_estimate_motion_b': /tmp/amv-ffmpeg/libavcodec/motion_est.c:1556: undefined reference to ff_epzs_motion_search'
/usr/bin/ld: /tmp/amv-ffmpeg/libavcodec/libavcodec.a(motion_est.o): in function direct_search': /tmp/amv-ffmpeg/libavcodec/motion_est.c:1819: undefined reference to ff_epzs_motion_search'
/usr/bin/ld: /tmp/amv-ffmpeg/libavcodec/libavcodec.a(motion_est.o): in function ff_estimate_p_frame_motion': /tmp/amv-ffmpeg/libavcodec/motion_est.c:1273: undefined reference to ff_epzs_motion_search'
/usr/bin/ld: /tmp/amv-ffmpeg/libavcodec/libavcodec.a(motion_est.o): in function ff_pre_estimate_p_frame_motion': /tmp/amv-ffmpeg/libavcodec/motion_est.c:1473: undefined reference to ff_epzs_motion_search'
/usr/bin/ld: /tmp/amv-ffmpeg/libavcodec/libavcodec.a(snow.o): in function encode_q_branch': /tmp/amv-ffmpeg/libavcodec/snow.c:1892: undefined reference to ff_epzs_motion_search'
/usr/bin/ld: /tmp/amv-ffmpeg/libavcodec/snow.c:1901: undefined reference to ff_get_mb_score' /usr/bin/ld: /tmp/amv-ffmpeg/libavcodec/snow.c:1892: undefined reference to ff_epzs_motion_search'
/usr/bin/ld: /tmp/amv-ffmpeg/libavcodec/snow.c:1901: undefined reference to `ff_get_mb_score'
collect2: error: ld returned 1 exit status

Adding extern to inline functions ff_epzs_motion_search and ff_get_mb_score in libavcodec/mpegvideo.h declarations seems to solve the problem.