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>
