Fix error: array subscript has type 'char'

Use unsigned char variable, or cast to (unsigned char), for
tolower()/islower() and friends. Fix compiler error
array subscript has type 'char' [-Werror=char-subscripts]

Signed-off-by: Manuel Bouyer <bouyer@netbsd.org>
Reviewed-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/libs/light/libxl_qmp.c

@@ -1249,7 +1249,7 @@ static int qmp_error_class_to_libxl_error_code(libxl__gc *gc,
                 se++;
                 continue;
             }
-            if (tolower(*s) != tolower(*se))
+            if (tolower((unsigned char)*s) != tolower((unsigned char)*se))
                 break;
             s++, se++;
         }

tools/xentrace/xentrace.c

@@ -957,7 +957,7 @@ static int parse_cpumask_range(const char *mask_str, xc_cpumap_t map)
 {
     unsigned int a, b;
     int nmaskbits;
-    char c;
+    unsigned char c;
     int in_range;
     const char *s;
 

xen/tools/symbols.c

@@ -173,11 +173,11 @@ static int read_symbol(FILE *in, struct sym_entry *s)
 	/* include the type field in the symbol name, so that it gets
 	 * compressed together */
 	s->len = strlen(str) + 1;
-	if (islower(stype) && filename)
+	if (islower((unsigned char)stype) && filename)
 		s->len += strlen(filename) + 1;
 	s->sym = malloc(s->len + 1);
 	sym = SYMBOL_NAME(s);
-	if (islower(stype) && filename) {
+	if (islower((unsigned char)stype) && filename) {
 		sym = stpcpy(sym, filename);
 		*sym++ = '#';
 	}