Correctly set `qt_ver=6' for Qt6 in Linux (#775)

This is a follow up to 155ad0e.

There was a bug that GYP variable `qt_ver=6' can never be set in Linux.
With this commit `qt_ver=6' will be correctly set for Qt6.

PiperOrigin-RevId: 556545280

src/build_mozc.py

@@ -482,7 +482,7 @@ def GypMain(options, unused_args):
     if target_platform == 'Linux':
       if PkgExists('Qt6Core', 'Qt6Gui', 'Qt6Widgets'):
         qt_ver = 6
-      if PkgExists('Qt5Core', 'Qt5Gui', 'Qt5Widgets'):
+      elif PkgExists('Qt5Core', 'Qt5Gui', 'Qt5Widgets'):
         qt_ver = 5
       else:
         PrintErrorAndExit('Qt is required to build GUI Tool. '