xen/param: address violation of MISRA C Rule 20.7

The rule states: Expressions resulting from the expansion of
macro parameters shall be enclosed in parentheses.

Add parentheses in the defition of `custom_runtime_var_sz' to
address the violations.

No functional change.

Signed-off-by: Nicola Vetrini <nicola.vetrini@gmail.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/include/xen/param.h

@@ -102,8 +102,8 @@ extern struct param_hypfs __paramhypfs_start[], __paramhypfs_end[];
 
 #define custom_runtime_set_var_sz(parfs, var, sz) \
     { \
-        (parfs)->hypfs.u.content = var; \
-        (parfs)->hypfs.e.max_size = sz; \
+        (parfs)->hypfs.u.content = (var); \
+        (parfs)->hypfs.e.max_size = (sz); \
         (parfs)->hypfs.e.size = strlen(var) + 1; \
     }
 #define custom_runtime_set_var(parfs, var) \