bktr: fix warning: missing braces around initializer

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavdevice/bktr.c

@@ -101,7 +101,7 @@ static av_cold int bktr_init(const char *video_device, int width, int height,
     long ioctl_frequency;
     char *arg;
     int c;
-    struct sigaction act = { 0 }, old;
+    struct sigaction act = { {0} }, old;
 
     if (idev < 0 || idev > 4)
     {