build: Let the iac decoder depend on the imc decoder

Almost all of the code is shared so there is little point in
enabling one decoder without the other.

configure

@@ -1694,7 +1694,7 @@ h264_decoder_suggest="error_resilience"
 hevc_decoder_select="dsputil golomb videodsp"
 huffyuv_decoder_select="dsputil"
 huffyuv_encoder_select="dsputil huffman"
-iac_decoder_select="dsputil fft mdct sinewin"
+iac_decoder_select="imc_decoder"
 imc_decoder_select="dsputil fft mdct sinewin"
 indeo3_decoder_select="hpeldsp"
 interplay_video_decoder_select="hpeldsp"

libavcodec/Makefile

@@ -204,7 +204,6 @@ OBJS-$(CONFIG_HEVC_DECODER)            += hevc.o hevc_mvs.o hevc_ps.o hevc_sei.o
 OBJS-$(CONFIG_HNM4_VIDEO_DECODER)      += hnm4video.o
 OBJS-$(CONFIG_HUFFYUV_DECODER)         += huffyuv.o huffyuvdec.o
 OBJS-$(CONFIG_HUFFYUV_ENCODER)         += huffyuv.o huffyuvenc.o
-OBJS-$(CONFIG_IAC_DECODER)             += imc.o
 OBJS-$(CONFIG_IDCIN_DECODER)           += idcinvideo.o
 OBJS-$(CONFIG_IFF_BYTERUN1_DECODER)    += iff.o
 OBJS-$(CONFIG_IFF_ILBM_DECODER)        += iff.o