Add missing check when doing qpdf build

We need this, since 5.15 branch adds webrtc check.

Change-Id: I40b63bf38ed57562562305d34f9f5ce96dec31dd
Reviewed-by: Michael Brüning <michael.bruning@qt.io>

src/buildtools/config/linux.pri

@@ -196,7 +196,7 @@ host_build {
     }
     !packagesExist(libpci): gn_args += use_libpci=false
 
-    qtConfig(webengine-ozone-x11) {
+    qtConfig(build-qtwebengine-core):qtConfig(webengine-ozone-x11) {
         gn_args += ozone_platform_x11=true
         packagesExist(xscrnsaver): gn_args += use_xscrnsaver=true
     }