[components] fix dtb compiling warning.

components/drivers/fdt/src/dtb_base.c

@@ -14,7 +14,7 @@ static const char *_parse_integer_fixup_radix(const char *s, unsigned int *base)
     {
         if (s[0] == '0')
         {
-            if (tolower(s[1]) == 'x' && isxdigit(s[2]))
+            if (tolower(s[1]) == 'x' && isxdigit((int)s[2]))
                 *base = 16;
             else
                 *base = 8;
@@ -35,7 +35,7 @@ unsigned long simple_strtoul(const char *cp, char **endp,
 
     cp = _parse_integer_fixup_radix(cp, &base);
 
-    while (isxdigit(*cp) && (value = isdigit(*cp) ? *cp-'0' : (islower(*cp)
+    while (isxdigit((int)*cp) && (value = isdigit((int)*cp) ? *cp-'0' : (islower((int)*cp)
         ? toupper(*cp) : *cp)-'A'+10) < base)
     {
         result = result*base + value;