CMake: Make qt5_use_modules a macro, not a function.

CMake: Make qt5_use_modules a macro, not a function.

Because qt5_use_modules is a function that does a find package which
finds imported targets, things like this:

 qt5_use_modules(foo Sql)
 if (TARGET Qt5::Sql)
    message("Qt5Sql_FOUND: ${Qt5Sql_FOUND}")
 endif()

will show Qt5Sql as being not found, even though the target is in scope.

Fix that by making it a macro instead.

Change-Id: If314bd5b4d3f769c6c7df5ff2c924eabd16dcc0e
Reviewed-by: Alexander Neundorf <neundorf@kde.org>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>