Delete unnecessary 'extern' keywords.

ffmpeg.c

libavcodec/alpha/dsputil_alpha.c

libavcodec/armv4l/dsputil_arm.c

libavcodec/armv4l/float_arm_vfp.c

libavcodec/armv4l/mpegvideo_arm.c

libavcodec/armv4l/mpegvideo_armv5te.c

libavcodec/avcodec.h

libavcodec/bfin/dsputil_bfin.c

libavcodec/bfin/mpegvideo_bfin.c

libavcodec/bfin/vp3_bfin.c

libavcodec/celp_math.h

libavcodec/dct-test.c

libavcodec/i386/dsputil_mmx.c

libavcodec/libxvidff.c

libavcodec/mpc.h

libavcodec/mpeg12.c

libavcodec/mpegvideo.c

libavcodec/png.h

libavcodec/ppc/dsputil_altivec.h

libavcodec/ppc/dsputil_ppc.c