poppler: fix typo in variable name

As pointed out during the check-package discussion, there is a typo in a
variable name:

http://lists.busybox.net/pipermail/buildroot/2016-December/180765.html

Reported-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/poppler/poppler.mk

@@ -18,7 +18,7 @@ ifeq ($(BR2_PACKAGE_CAIRO),y)
 POPPLER_CONF_OPTS += --enable-cairo-output
 POPPLER_DEPENDENCIES += cairo
 else
-POPLER_CONF_OPTS += --disable-cairo-output
+POPPLER_CONF_OPTS += --disable-cairo-output
 endif
 
 ifeq ($(BR2_PACKAGE_LCMS2),y)