Fix unbundling of libpng in pdfium

It is a separate GN setting

Change-Id: I0fdd9c631a0f9dac3cf63cad4c1eb0c4b97786cb
Reviewed-by: Michael Brüning <michael.bruning@qt.io>

src/core/config/linux.pri

@@ -123,10 +123,14 @@ host_build {
         gn_args += host_pkg_config=\"$$PKG_CONFIG_HOST\"
     }
 
-    qtConfig(webengine-system-zlib): qtConfig(webengine-system-minizip) {
-        gn_args += use_system_zlib=true use_system_minizip=true
+    qtConfig(webengine-system-zlib) {
+        qtConfig(webengine-system-minizip): gn_args += use_system_zlib=true use_system_minizip=true
         qtConfig(webengine-printing-and-pdf): gn_args += pdfium_use_system_zlib=true
     }
+    qtConfig(webengine-system-png) {
+        gn_args += use_system_libpng=true
+        qtConfig(webengine-printing-and-pdf): gn_args += pdfium_use_system_libpng=true
+    }
     qtConfig(webengine-system-png): gn_args += use_system_libpng=true
     qtConfig(webengine-system-jpeg): gn_args += use_system_libjpeg=true
     qtConfig(system-freetype): gn_args += use_system_freetype=true