fix qtmain's .prl file not being used

at some point we stopped adding the qtmain's library path before its
respective -l flag, which lead to qmake being unable to resolve the
library location and thus ignoring its prl file.

Change-Id: I390a31f8ac2877d3823dfd2787b2cc8c696b0ec0
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>

mkspecs/features/win32/windows.prf

@@ -7,6 +7,8 @@ contains(TEMPLATE, ".*app"){
 
     qt:for(entryLib, $$list($$unique(QMAKE_LIBS_QT_ENTRY))) {
         isEqual(entryLib, -lqtmain): {
+            !contains(QMAKE_DEFAULT_LIBDIRS, $$QT.core.libs): \
+                QMAKE_LIBS += -L$$QT.core.libs
             CONFIG(debug, debug|release): QMAKE_LIBS += $${entryLib}$${QT_LIBINFIX}d
             else: QMAKE_LIBS += $${entryLib}$${QT_LIBINFIX}
         } else {