package/tvheadend: add support for host-python3

Added upstream:
https://github.com/tvheadend/tvheadend/commit/5a07ff166a84c65f08d326c69c818f6eef315e68



Tested with these defconfigs:

BR2_TOOLCHAIN_BUILDROOT_LARGEFILE=y
BR2_TOOLCHAIN_BUILDROOT_INET_IPV6=y
BR2_TOOLCHAIN_BUILDROOT_WCHAR=y
BR2_PACKAGE_PYTHON3=y
BR2_PACKAGE_TVHEADEND=y

BR2_TOOLCHAIN_BUILDROOT_LARGEFILE=y
BR2_TOOLCHAIN_BUILDROOT_INET_IPV6=y
BR2_TOOLCHAIN_BUILDROOT_WCHAR=y
BR2_PACKAGE_PYTHON=y
BR2_PACKAGE_TVHEADEND=y

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>