Windeployqt: change platform plugin base filter

Add infix to "qwindows" for platform plugin search if it exists, to
account for libinfix builds

Task-number: QTBUG-105820
Pick-to: 6.4
Change-Id: I7a3c671fb6cdb4a216f6f4bffb445add4f8e0c83
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>

src/tools/windeployqt/main.cpp

@@ -1024,6 +1024,8 @@ QStringList findQtPlugins(ModuleBitset *usedQtModules, const ModuleBitset &disab
                 case WindowsDesktopMsvc:
                 case WindowsDesktopMinGW:
                     filter = QStringLiteral("qwindows");
+                    if (!infix.isEmpty())
+                        filter += infix;
                     break;
                 case Unix:
                     filter = QStringLiteral("libqxcb");