asterisk-15.x: clean up DEPENDS

asterisk-15.x: clean up DEPENDS

- The main package does not actually depend on libpopt. Only the utility
  smsq does. This commit reflects that in the dependencies.

- libxslt is only used if generating the XML documentation, which this
  package does not use. So remove the bogus dependency.

- app-sms does not depend on libpopt either. It has another dependency
  on libstdcpp, which is already covered by the main package. This removes
  both dependencies.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>