FindQt4: Restore QT4_FOUND setting
Commit 1a8e43d57d (FindPackageHandleStandardArgs: Clarify the FOUND_VAR
option, 2024-12-31, v4.0.0-rc1~247^2) simplified the
`<PackageName>_FOUND` and `<PACKAGENAME>_FOUND` variables one step
further but missed that this specific module sets the `QT4_FOUND`
variable also for the case where `find_package_handle_standard_args()`
isn't called (for example, when the found package is Qt3 or Qt5 and
later). Restore the `set()` call in case `QT4_FOUND` is used in the
code.