fix enable-mpl-subset with --with-system-poppler

Change-Id: I2828d662769f95241ee5760bb657201851f3193e
Reviewed-on: https://gerrit.libreoffice.org/18209Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>

configure.ac

@@ -12972,7 +12972,7 @@ if test "$enable_mpl_subset" = "yes"; then
         AC_MSG_ERROR([need to --disable-ext-mariadb-connector - mariadb/mysql support.])
     fi
     if test -n "$ENABLE_PDFIMPORT"; then
-        if test "x$SYSTEM_POPPLER" != "x"; then
+        if test "x$SYSTEM_POPPLER" = "x"; then
             AC_MSG_ERROR([need to disable PDF import via poppler or use system library])
         fi
     fi