avformat/libssh: Fix libssh defaulting to shared linkage.

Reviewed-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Reviewed-by: Lukasz M <lukasz.m.luki@gmail.com>
See: [FFmpeg-devel] Fix libssh static linkage on Windows
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/libssh.c

@@ -19,6 +19,7 @@
  */
 
 #include <fcntl.h>
+#define LIBSSH_STATIC
 #include <libssh/sftp.h>
 #include "libavutil/avstring.h"
 #include "libavutil/opt.h"