Correct upper boundary for isxdigit.

Signed-off-by: Ulf Jordan <jordan@chalmers.se>

Index: libc/ctype.c
===================================================================
--- libc/ctype.c	(revision 3237)
+++ libc/ctype.c	(arbetskopia)
@@ -91,7 +91,7 @@
 
 int isxdigit(int c)
 {
-	return isdigit(c) || (tolower(c) >= 'a' && tolower(c) <= 'z');
+	return isdigit(c) || (tolower(c) >= 'a' && tolower(c) <= 'f');
 }
 
 int tolower(int c)