lavf/tls_gnutls: fix warnings from version check

The GnuTLS version is checked through the macro GNUTLS_VERSION_NUMBER,
but this wasn't introduced before 2.7.2. Building with older versions
of GnuTLS (using icc) warns:

src/libavformat/tls_gnutls.c(38): warning #193: zero used for undefined preprocessing identifier "GNUTLS_VERSION_NUMBER"
  #if HAVE_THREADS && GNUTLS_VERSION_NUMBER <= 0x020b00

This adds a fallback to the older, deprecated LIBGNUTLS_VERSION_NUMBER
macro.
Signed-off-by: Moritz Barsnick <barsnick@gmx.net>

libavformat/tls_gnutls.c

@@ -35,6 +35,10 @@
 #include "libavutil/opt.h"
 #include "libavutil/parseutils.h"
 
+#ifndef GNUTLS_VERSION_NUMBER
+#define GNUTLS_VERSION_NUMBER LIBGNUTLS_VERSION_NUMBER
+#endif
+
 #if HAVE_THREADS && GNUTLS_VERSION_NUMBER <= 0x020b00
 #include <gcrypt.h>
 #include "libavutil/thread.h"