poppler: Install files into staging

Install poppler's header files and libraries into staging, since it
provides a library that can be used by other packages.

[Thomas: slightly extended the commit message.]

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>