Revert "QRCodeGen: Exclude unused library from build"

This reverts commit 2f66842206.

Reason for revert: QrCodeGenerator is now unconditionally used in
QmlDesigner plugin (merged in commit e606909c38)

Change-Id: I7b47dc89611fa1ea27322dc4463da87398c548a2
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>

src/libs/3rdparty/CMakeLists.txt

@@ -2,6 +2,7 @@ add_subdirectory(cplusplus)
 add_subdirectory(syntax-highlighting)
 add_subdirectory(libvterm)
 add_subdirectory(libptyqt)
+add_subdirectory(qrcodegen)
 add_subdirectory(qtkeychain)
 add_subdirectory(lua)
 add_subdirectory(sol2)
@@ -10,7 +11,3 @@ add_subdirectory(karchive)
 if(WIN32)
     add_subdirectory(winpty)
 endif()
-
-if (BUILD_DESIGNSTUDIO)
-    add_subdirectory(qrcodegen)
-endif()