random: move random_min_urandom_seed into CONFIG_SYSCTL ifdef block

Building arm allnodefconfig causes the following build warning:

drivers/char/random.c:318:12: warning: 'random_min_urandom_seed' defined but not used [-Wunused-variable]

Fix the warning by moving 'random_min_urandom_seed' declaration inside
the CONFIG_SYSCTL ifdef block, where it is actually used.

While at it, remove the comment prior to the variable declaration.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>