We must defined all members to NULL (select, waitpid, ...)

In file included from gnutls.c:63:
../../include/vlc_gcrypt.h:81: warning: missing initializer
../../include/vlc_gcrypt.h:81: warning: (near initialization for 'gcry_threads_vlc.read')
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>