lavfi/Makefile: fix vf_cropdetect missed edge_common

vf_cropdetect depends on edge_common, it's missing in Makefile.

Fix trac issue:
http://trac.ffmpeg.org/ticket/10664

Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

libavfilter/Makefile

@@ -255,7 +255,7 @@ OBJS-$(CONFIG_COREIMAGE_FILTER)              += vf_coreimage.o
 OBJS-$(CONFIG_CORR_FILTER)                   += vf_corr.o framesync.o
 OBJS-$(CONFIG_COVER_RECT_FILTER)             += vf_cover_rect.o lavfutils.o
 OBJS-$(CONFIG_CROP_FILTER)                   += vf_crop.o
-OBJS-$(CONFIG_CROPDETECT_FILTER)             += vf_cropdetect.o
+OBJS-$(CONFIG_CROPDETECT_FILTER)             += vf_cropdetect.o edge_common.o
 OBJS-$(CONFIG_CUE_FILTER)                    += f_cue.o
 OBJS-$(CONFIG_CURVES_FILTER)                 += vf_curves.o
 OBJS-$(CONFIG_DATASCOPE_FILTER)              += vf_datascope.o