man-pages: add an alternative link name for crypt_r.3

Because crypt_r.3 is also provided by libxcrypt-doc.

Signed-off-by: Thomas Perrot <thomas.perrot@bootlin.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>