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>

package/poppler/poppler.mk

@@ -11,7 +11,8 @@ POPPLER_DEPENDENCIES = fontconfig host-pkgconf
 POPPLER_LICENSE = GPLv2+
 POPPLER_LICENSE_FILES = COPYING
 POPPLER_INSTALL_STAGING = YES
-POPPLER_CONF_OPTS = --with-font-configuration=fontconfig
+POPPLER_CONF_OPTS = --with-font-configuration=fontconfig \
+	--enable-xpdf-headers
 
 ifeq ($(BR2_PACKAGE_LCMS2),y)
 POPPLER_CONF_OPTS += --enable-cms=lcms2