avdevice/decklink_dec: mark get_frame_timecode and get_bmd_timecode static

The function is not used anywhere else and is causing mingw-w64 clang
builds to fail with

ffmpeg-git/libavdevice/decklink_dec.cpp:792:5: error: no previous prototype for function 'get_bmd_timecode' [-Werror,-Wmissing-prototypes]
int get_bmd_timecode(AVFormatContext *avctx, AVTimecode *tc, AVRational frame_rate, BMDTimecodeFormat tc_format, IDeckLinkVideoInputFrame *videoFrame)
Signed-off-by: Christopher Degawa <ccom@randomderp.com>
Signed-off-by: Marton Balint <cus@passwd.hu>

libavdevice/decklink_dec.cpp

@@ -789,7 +789,7 @@ static int64_t get_pkt_pts(IDeckLinkVideoInputFrame *videoFrame,
     return pts;
 }
 
-int get_bmd_timecode(AVFormatContext *avctx, AVTimecode *tc, AVRational frame_rate, BMDTimecodeFormat tc_format, IDeckLinkVideoInputFrame *videoFrame)
+static int get_bmd_timecode(AVFormatContext *avctx, AVTimecode *tc, AVRational frame_rate, BMDTimecodeFormat tc_format, IDeckLinkVideoInputFrame *videoFrame)
 {
     IDeckLinkTimecode *timecode;
     int ret = AVERROR(ENOENT);
@@ -811,7 +811,7 @@ int get_bmd_timecode(AVFormatContext *avctx, AVTimecode *tc, AVRational frame_ra
     return ret;
 }
 
-int get_frame_timecode(AVFormatContext *avctx, decklink_ctx *ctx, AVTimecode *tc, IDeckLinkVideoInputFrame *videoFrame)
+static int get_frame_timecode(AVFormatContext *avctx, decklink_ctx *ctx, AVTimecode *tc, IDeckLinkVideoInputFrame *videoFrame)
 {
     AVRational frame_rate = ctx->video_st->r_frame_rate;
     int ret;