avcodec/Makefile: Only compile nvenc.o if needed

This fixes compilation errors in case nvenc is enabled
(e.g. autodected) with both nvenc-based encoders disabled
because nvenc uses ff_alloc_a53_sei(), yet only the nvenc-based
encoders require atsc_a53.
(This error does not manifest itself in case of static linking
(nothing pulls in nvenc.o), but it exists with shared builds.)

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
Signed-off-by: Timo Rothenpieler <timo@rothenpieler.org>

libavcodec/Makefile

@@ -135,7 +135,6 @@ OBJS-$(CONFIG_MPEGVIDEOENC)            += mpegvideo_enc.o mpeg12data.o  \
                                           motion_est.o ratecontrol.o    \
                                           mpegvideoencdsp.o
 OBJS-$(CONFIG_MSS34DSP)                += mss34dsp.o
-OBJS-$(CONFIG_NVENC)                   += nvenc.o
 OBJS-$(CONFIG_PIXBLOCKDSP)             += pixblockdsp.o
 OBJS-$(CONFIG_QPELDSP)                 += qpeldsp.o
 OBJS-$(CONFIG_QSV)                     += qsv.o
@@ -383,7 +382,7 @@ OBJS-$(CONFIG_H264_CUVID_DECODER)      += cuviddec.o
 OBJS-$(CONFIG_H264_MEDIACODEC_DECODER) += mediacodecdec.o
 OBJS-$(CONFIG_H264_MF_ENCODER)         += mfenc.o mf_utils.o
 OBJS-$(CONFIG_H264_MMAL_DECODER)       += mmaldec.o
-OBJS-$(CONFIG_H264_NVENC_ENCODER)      += nvenc_h264.o
+OBJS-$(CONFIG_H264_NVENC_ENCODER)      += nvenc_h264.o nvenc.o
 OBJS-$(CONFIG_H264_OMX_ENCODER)        += omx.o
 OBJS-$(CONFIG_H264_QSV_DECODER)        += qsvdec.o
 OBJS-$(CONFIG_H264_QSV_ENCODER)        += qsvenc_h264.o
@@ -404,7 +403,7 @@ OBJS-$(CONFIG_HEVC_AMF_ENCODER)        += amfenc_hevc.o
 OBJS-$(CONFIG_HEVC_CUVID_DECODER)      += cuviddec.o
 OBJS-$(CONFIG_HEVC_MEDIACODEC_DECODER) += mediacodecdec.o
 OBJS-$(CONFIG_HEVC_MF_ENCODER)         += mfenc.o mf_utils.o
-OBJS-$(CONFIG_HEVC_NVENC_ENCODER)      += nvenc_hevc.o
+OBJS-$(CONFIG_HEVC_NVENC_ENCODER)      += nvenc_hevc.o nvenc.o
 OBJS-$(CONFIG_HEVC_QSV_DECODER)        += qsvdec.o
 OBJS-$(CONFIG_HEVC_QSV_ENCODER)        += qsvenc_hevc.o hevc_ps_enc.o       \
                                           hevc_data.o