package/tvheadend: Rework CFLAGS handling for liburiparser

Suggested by Thomas:
http://article.gmane.org/gmane.comp.lib.uclibc.buildroot/100646



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>