xen/sched: address violations of MISRA C Rule 20.7

MISRA C Rule 20.7 states: "Expressions resulting from the expansion
of macro parameters shall be enclosed in parentheses". Therefore, some
macro definitions should gain additional parentheses to ensure that all
current and future users will be safe with respect to expansions that
can possibly alter the semantics of the passed-in macro parameter.

No functional change.

Signed-off-by: Nicola Vetrini <nicola.vetrini@bugseng.com>
Acked-by: George Dunlap <george.dunlap@cloud.com>

xen/common/sched/private.h

@@ -540,7 +540,7 @@ static inline void sched_unit_unpause(const struct sched_unit *unit)
 }
 
 #define REGISTER_SCHEDULER(x) static const struct scheduler *x##_entry \
-  __used_section(".data.schedulers") = &x
+  __used_section(".data.schedulers") = &(x)
 
 struct cpupool
 {