gcrypt: prefer the OS RNG

by default, gcrypt defaults to an userspace RNG, this is
the wrong thing (tm) to do on linux.

Switch to the SYSTEM rng instead.

(cherry picked from commit 80f9673)

src/basic/gcrypt-util.c

@@ -15,6 +15,8 @@ void initialize_libgcrypt(bool secmem) {
          * feature should initialize the library manually */
         if (!secmem)
                 gcry_control(GCRYCTL_DISABLE_SECMEM);
+
+        gcry_control(GCRYCTL_SET_PREFERRED_RNG_TYPE, GCRY_RNG_TYPE_SYSTEM);
         gcry_control(GCRYCTL_INITIALIZATION_FINISHED, 0);
 }