package/libepoxy: needs threads

Buildroot commit cd2583ca0a993cb44e8822966f101e820ac2619e removed the
dependencies to gl/egl which revealed that libepoxy needs a toolchain
with threads:

../src/dispatch_common.c:168:10: fatal error: pthread.h:
 No such file or directory
  168 | #include <pthread.h>

Fixes:
https://autobuild.buildroot.net/results/dc2/dc27a008af73c7676e4f54e4846d87f9d5aef98e/

Signed-off-by: Bernd Kuhls <bernd@kuhls.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>