package/cups-filters: fix build without dejavu font

package/cups-filters: fix build without dejavu font

Since version 1.27.3, cups-filters needs dejavu (even if it is only used
for test programs):
https://github.com/OpenPrinting/cups-filters/commit/1d66106e5ae45407b01459cb112ee09752166dba

Add a patch to avoid this build failure when cross-compiling and set
test font path to /dev/null to avoid setting TESTFONT to an incorrect
host path

Fixes:
 - http://autobuild.buildroot.org/results/0e141abc57e4185c74adce75cac4215dd8a3108b

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