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>

package/poppler/poppler.mk

@@ -10,6 +10,7 @@ POPPLER_SITE = http://poppler.freedesktop.org
 POPPLER_DEPENDENCIES = fontconfig
 POPPLER_LICENSE = GPLv2+
 POPPLER_LICENSE_FILES = COPYING
+POPPLER_INSTALL_STAGING = YES
 POPPLER_CONF_OPTS = --with-font-configuration=fontconfig
 
 ifeq ($(BR2_PACKAGE_LCMS2),y)