package/freeradius-server: fix memcached dependency

Fix the following build failure raised since the addition of the package
in commit 736c4c16:

Makefile:575: *** mysql is in the dependency chain of freeradius-server that has added it to its _DEPENDENCIES variable without selecting it or depending on it from Config.in.  Stop.

Fixes:
 - http://autobuild.buildroot.org/results/c3a9c3822b4599c1b16d27840b5663506e37a41f



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>