rbd: don't link with -lcrypto

rbd support tries to both link with -lrados and -lcrypto. While the
first one is of course necessary, the second is not necessary (only
librados ifself needs to link with libcrypto).

This fixes a licensing issue: qemu as a whole is GPL v2, and thus can't
be linked with OpenSSL without an exception in the license, which seems
difficult to get given the number of persons involved.

Cc: Christian Brunner <chb@muc.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
(cherry picked from commit cc4e8741)