qpdfview: fix build

With structuredAttrs support in 1bcca7d66b
this broke, because the following line turns qmakeFlags from a string
into an array:

  qmakeFlags+=(...)

It will then be handled slightly different when it's an array, so that
all the xxx_INSTALL_PATH variables are passed as a *single* argument,
which breaks the build.

Resolves #342087

pkgs/applications/office/qpdfview/default.nix

@@ -40,6 +40,9 @@ mkDerivation rec {
     ghostscript
   ];
 
+  # needed for qmakeFlags+=( below
+  __structuredAttrs = true;
+
   preConfigure = ''
     lrelease qpdfview.pro
     qmakeFlags+=(*.pro)