Correct formatting of gcry_mac_get_algo_keylen documentation

* doc/gcrypt.texi: add braces near gcry_mac_get_algo_keylen
  documentation.

Use braces around unsigned int in gcry_mac_get_algo_keylen
documentation, otherwise texinfo breaks that and uses 'int' as a
function definition.

• Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>