Add [[maybe_unused]] to avoid compilation warnings on Android

When cross compiling for Android, these need to be marked with
[[maybe_unused]] as they might break the build due to Android not having
pthread cancellation feature.

Pick-to: 6.5
Change-Id: I3b3fb58b5e6df8ce37c2d8d9b1b8e8295f0aebb0
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qprocess_unix.cpp

@@ -529,7 +529,7 @@ void QProcessPrivate::startProcess()
     // Disable PThread cancellation from this point on: we mustn't have it
     // enabled when the child starts running nor while our state could get
     // corrupted if we abruptly exited this function.
-    PThreadCancelGuard cancelGuard;
+    [[maybe_unused]] PThreadCancelGuard cancelGuard;
 
     // Start the child.
     auto execChild1 = [this, workingDirFd, &argv, &envp]() {
@@ -1088,7 +1088,7 @@ bool QProcessPrivate::startDetached(qint64 *pid)
     const CharPointerList envp(environment.d.constData());
 
     // see startProcess() for more information
-    PThreadCancelGuard cancelGuard;
+    [[maybe_unused]] PThreadCancelGuard cancelGuard;
 
     auto doFork = [this]() {
         if (useForkFlags(unixExtras.get()))