gnu: qt: Set 'EffectiveSourcePaths' to 'qtbase' for submodules.

Fixes a regression introduced in 8075b62378.
This allows 'linguist.pro' of 'qttools' to find 'ConfigVersion.cmake.in',
which is needed for 'Qt5LinguistToolsConfigVersion.cmake'.

Reported by Thomas Danckaert <post@thomasdanckaert.be>.

* gnu/packages/qt.scm (qtsvg)[arguments]: Set 'EffectiveSourcePaths' in
the 'configure-qmake' phase.

gnu/packages/qt.scm

@@ -617,7 +617,11 @@ HostPrefix=~a
 HostData=lib/qt5
 HostBinaries=bin
 HostLibraries=lib
-" out out)))
+
+[EffectiveSourcePaths]
+HostPrefix=~a
+HostData=lib/qt5
+" out out qtbase)))
                #t)))
          (replace 'configure
            (lambda* (#:key inputs outputs #:allow-other-keys)