include/ctype.h: fix MISRA R10.2 violation

The value returned by __toupper is used in arithmetic operations causing
MISRA C 10.2 violations. Cast to plain char in the toupper macro. Also
do the same in tolower for consistency.

Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/include/xen/ctype.h

@@ -48,7 +48,7 @@ static inline unsigned char __toupper(unsigned char c)
 	return c;
 }
 
-#define tolower(c) __tolower(c)
-#define toupper(c) __toupper(c)
+#define tolower(c) ((char)__tolower(c))
+#define toupper(c) ((char)__toupper(c))
 
 #endif