poppler: install xpdf-headers to enable building of cups-filters

Add --enable-xpdf-headers to poppler.mk. Required for succesfull
build of cups-filters.

Signed-off-by: Olivier Schonken <olivier.schonken@gmail.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>