package/usbguard: needs at least a crypto backend

package/usbguard: needs at least a crypto backend

Since its addition in commit fbff7d7289cc95db991184f890f4ca1fcf8a101e,
usbguard needs a crypto backend otherwise the build will fail on:

checking for sodium... no
checking for libcrypto... no
checking for powerpc-buildroot-linux-uclibc-libgcrypt-config... no
checking for libgcrypt-config... no
checking for LIBGCRYPT - version >= 1.5.0... no
configure: error: The selected crypto backend library is not available.

So select sodium as the default crypto backend as this is the default
choice in configure.ac.

Fixes:
 - http://autobuild.buildroot.org/results/fb9865e2c65ea1dd9a657e1181a6c8c49481fda7

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>