avformat/segafilmenc: Fix undefined left shift of 1 by 31 places

by changing the type to unsigned.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/segafilmenc.c

@@ -70,7 +70,7 @@ static int film_write_packet_to_header(AVFormatContext *format_context, FILMPack
         info2 = pkt->duration;
         /* The top bit being set indicates a key frame */
         if (!pkt->keyframe)
-            info1 |= (1 << 31);
+            info1 |= 1U << 31;
     }
 
     /* Write the 16-byte sample info packet to the STAB chunk in the header */