build: fix standalone compilation of ADX encoder

The encoder depends on the common code, so link against it.

libavcodec/Makefile

@@ -493,7 +493,7 @@ OBJS-$(CONFIG_PCM_ZORK_DECODER)           += pcm.o
 
 OBJS-$(CONFIG_ADPCM_4XM_DECODER)          += adpcm.o adpcm_data.o
 OBJS-$(CONFIG_ADPCM_ADX_DECODER)          += adxdec.o adx.o
-OBJS-$(CONFIG_ADPCM_ADX_ENCODER)          += adxenc.o
+OBJS-$(CONFIG_ADPCM_ADX_ENCODER)          += adxenc.o adx.o
 OBJS-$(CONFIG_ADPCM_CT_DECODER)           += adpcm.o adpcm_data.o
 OBJS-$(CONFIG_ADPCM_EA_DECODER)           += adpcm.o adpcm_data.o
 OBJS-$(CONFIG_ADPCM_EA_MAXIS_XA_DECODER)  += adpcm.o adpcm_data.o