avcodec/nvenc: Make pix_fmts_nvenc const

Reviewed-by: Timo Rothenpieler <timo@rothenpieler.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/nvenc.c

@@ -1375,7 +1375,7 @@ static int nvenc_encode_frame(AVCodecContext *avctx, AVPacket *pkt,
     return 0;
 }
 
-static enum AVPixelFormat pix_fmts_nvenc[] = {
+static const enum AVPixelFormat pix_fmts_nvenc[] = {
     AV_PIX_FMT_NV12,
     AV_PIX_FMT_NONE
 };