package/poppler: add missing end-of-line backslash

package/poppler: add missing end-of-line backslash

Commit c6bef18dab5d "bump version to 0.77.0" converted the
POPPLER_CONF_OPTS variable to one entry per line, but a backslash was
forgotten at the end of one of the lines. Thus, the last bit of the
configuration options weren't applied.

Fortunately, all of those were alread at their default value, so not
much was lost.

It does fix a check-package warning though:
package/poppler/poppler.mk:17: unexpected indent with tabs

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>