Fix building libopencore_amrnb decoder without libopencore_amrnb encoder

Fixes #1210.
Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavcodec/Makefile

@@ -661,7 +661,6 @@ OBJS-$(CONFIG_LIBOPENCORE_AMRNB_DECODER)  += libopencore-amr.o \
                                              audio_frame_queue.o
 OBJS-$(CONFIG_LIBOPENCORE_AMRNB_ENCODER)  += libopencore-amr.o \
                                              audio_frame_queue.o
-OBJS-$(CONFIG_LIBOPENCORE_AMRWB_DECODER)  += libopencore-amr.o
 OBJS-$(CONFIG_LIBOPENJPEG_DECODER)        += libopenjpegdec.o
 OBJS-$(CONFIG_LIBOPENJPEG_ENCODER)        += libopenjpegenc.o
 OBJS-$(CONFIG_LIBSCHROEDINGER_DECODER)    += libschroedingerdec.o \