fontconfig: needs host-gperf

fontconfig: needs host-gperf

Commit 701380c53b4d("fontconfig: add build fix for glibc 2.25") added
a patch that touches src/fcobjs.h. Package build system detects change
in timestamp and tries to regenerate fcobjshash.gperf which requires
gperf tool. So this commit adds dependency on host-gperf.

Since the patch is in upstream fontconfig, we can get rid of this
dependency once version is bumped next time.

Fixes:
  http://autobuild.buildroot.net/results/174/17471088b26ebc840d1922d7f2aa9e69b4b49ced

Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Rahul Bedarkar <rahul.bedarkar@imgtec.com>
[Thomas: improve comment in the code.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>