Having "select PACKAGE_zoneinfo-core" wasn't enough for builds without
php7-cli=y or php7-cgi=y. It didn't result in installing zoneinfo-core
when using "opkg install" (during runtime or when building images with
CONFIG_TARGET_PER_DEVICE_ROOTFS).

Missing zoneinfo results in PHP fatal errors, e.g.:
Fatal error: DateTime::createFromFormat(): Timezone database is corrupt - this should *never* happen!

For years users were told to manually install zoneinfo-core package.
This problem was hidden for some time (including 17.01 release) due to
disabled support for CONFIG_PHP7_SYSTEMTZDATA. It's now back as support
for --with-system-tzdata was enabled again.

The proper solution is to simply make php7 package depend on
zoneinfo-core when PHP7_SYSTEMTZDATA is used.

Fixes: 84e5012e88 ("php7: re-enable system timezone data usage")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>