tidy: Explicitly test the return value of calling myth_system.

The clang-tidy "implicit boolean conversion" check pointed out one
spot where the return value of myth_system was implicitly converted
into a boolean result.  Fix to explicitly check the return value
against zero.  Changes made by the clang-tidy program.

https://clang.llvm.org/extra/clang-tidy/checks/readability-implicit-bool-conversion.html

mythtv/libs/libmythbase/mythmiscutil.cpp

@@ -660,7 +660,7 @@ bool WakeOnLAN(QString MAC)
 bool MythWakeup(const QString &wakeUpCommand, uint flags, uint timeout)
 {
     if (!IsMACAddress(wakeUpCommand))
-        return !myth_system(wakeUpCommand, flags, timeout);
+        return myth_system(wakeUpCommand, flags, timeout) == 0U;
 
     return WakeOnLAN(wakeUpCommand);
 }