QProcess: make UnixProcessFlag an enum class

See: https://lists.qt-project.org/pipermail/development/2023-May/043804.html

Not being an enum class looks more of an oversight, in most places usage
of the enumerators was already prefixed with QProcess::UnixProcessFlag.

This is cherry picked from 22c540a66dbb849bf6b8bf49027cfaf8510ef066, but
much smaller.

Change-Id: Ie37d74e0039d3f65f90af560cb85bb11b77ae20c
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 22c540a66dbb849bf6b8bf49027cfaf8510ef066)

src/corelib/io/qprocess.h

@@ -175,7 +175,7 @@ public:
     std::function<void(void)> childProcessModifier() const;
     void setChildProcessModifier(const std::function<void(void)> &modifier);
 
-    enum UnixProcessFlag : quint32 {
+    enum class UnixProcessFlag : quint32 {
         ResetSignalHandlers                 = 0x0001, // like POSIX_SPAWN_SETSIGDEF
         IgnoreSigPipe                       = 0x0002,
         // some room if we want to add IgnoreSigHup or so