init-repository.pl: Warn that the script is deprecated

The CMake port of init-repository supports every option of the Perl
script. Print a warning when init-repository.pl is run, telling users
to use init-repository instead, so that the script can be removed in a
later Qt version. The script keeps working.

Change-Id: I2693ad3df63b47d46c374c6082e1d27609211d04
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>