package/poppler: set g-ir-scanner/g-ir-compiler paths

package/poppler: set g-ir-scanner/g-ir-compiler paths

The g-ir-scanner/g-ir-compiler paths don't always get picked up
properly by cmake.

Fixes:
[ 98%] Linking CXX shared library libpoppler-glib.so
[ 98%] Built target poppler-glib
[ 98%] Generating Poppler-0.18.gir
/bin/sh: 1: /home/buildroot/buildroot/output/per-package/poppler/host/aarch64-buildroot-linux-gnu/sysroot/home/buildroot/buildroot/output/per-package/poppler/host/bin/../aarch64-buildroot-linux-gnu/sysroot/usr/lib/pkgconfig/../../bin/g-ir-scanner: not found

Signed-off-by: James Hilliard <[email protected]>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <[email protected]>