rtmpdh: Initialize gcrypt before using it

Either disabling or init'ing secure memory is required after the use
of gcry_check_version. From a look at the functions rtmpdh uses, I
noticed none require the use of secure memory, so we disable it [1][2].

This resolves some errors returned by rtmpdh code with uninitialized
gcrypt, especifically:
Fatal: failed to create the RNG lock: Invalid argument
FATAL: failed to acquire the FSM lock in libgrypt: Invalid argument

Version "1.5.4" was arbitrarily chosen. An older version probably works
as well, but I couldn't compile older versions to test on my machine.

[1]
https://gnupg.org/documentation/manuals/gcrypt/Initializing-the-library.html
[2]
https://www.gnupg.org/documentation/manuals/gcrypt/Controlling-the-library.html

Signed-off-by: Ricardo Constantino <wiiaboo@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
