package/freeradius-client: add libxcrypt optional dependency

package/freeradius-client: add libxcrypt optional dependency

Fix the following build failure raised since bump of glibc to version
2.39 in commit b5680f53d60acf8ff6010082f873438a39bd5d97:

/home/autobuild/autobuild/instance-13/output-1/host/lib/gcc/riscv32-buildroot-linux-gnu/12.3.0/../../../../riscv32-buildroot-linux-gnu/bin/ld: local.o: in function `auth_local':
local.c:(.text+0xe8): undefined reference to `crypt'

Fixes: b5680f53d60acf8ff6010082f873438a39bd5d97
 - http://autobuild.buildroot.org/results/207bba3df700108839382007020be56c4629ef75

Signed-off-by: Fabrice Fontaine <[email protected]>
Signed-off-by: Yann E. MORIN <[email protected]>