gst1-plugins-bad: fix broken dependency for the hls plugin

HLS plugin can be built with nettle or libgcrypt or openssl
cryptographic backend. But current dependency on gnutls is incorrect.
It has been working so far because gnutls depends on nettle.

gst-plugins-bad's build system for HLS allows user to choose which
cryptographic backend to use. If that is not specified, it internally
checks for nettle or libgcrypt or openssl in order. If none of the
cryptographic backend is available, HLS plugin gets disabled internally.

Select cryptographic backend according to which cryptographic packages
are available. If both libgcrypt or openssl are not available, choose
nettle by default.

Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Reviewed-by: Marcin Nowakowski <marcin.nowakowski@imgtec.com>
Signed-off-by: Rahul Bedarkar <rahul.bedarkar@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>