poppler: Add option for Qt support

Add an option to build the libpoppler-qt4 library.

[Thomas: slightly adjust the Config.in file, by:
  - Using an if BR2_PACKAGE_POPPLER ... endif block instead of a depends
    on BR2_PACKAGE_POPPLER for the new option.
  - Rewording the option prompt, since there's no need to indicate that
    the Qt support is for poppler since we're already a sub-option of
    poppler.]

Signed-off-by: Alan Ott <alan@signal11.us>
Suggested-by: Jeremie Scheer <jeremie.scheer@armadeus.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

merge