package/usbguard: fix build with libgcrypt

package/usbguard: fix build with libgcrypt

Building with libgcrypt fails since the addition of the package in
commit fbff7d7289cc95db991184f890f4ca1fcf8a101e:

checking for sparc64-buildroot-linux-gnu-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.

Fix this build failure by helping usbguard to find libgcrypt-config

Fixes:
 - http://autobuild.buildroot.org/results/4c19e50a2a5308cb31a62f1b4b538a30353022bb

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