string: correct prototype of strchrnul()

Both glibc's (where this originated as a GNU extension) and the
kernel's versions of strchrnul() return "char *", not "const
char *". That also makes it consistent with the standard strchr()
function.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Rasmus Villemoes <rv@rasmusvillemoes.dk>

include/linux/string.h

@@ -63,7 +63,7 @@ extern char * strchr(const char *,int);
  * @c: character to search for
  * Return: position of @c in @s, or end of @s if not found
  */
-const char *strchrnul(const char *s, int c);
+char *strchrnul(const char *s, int c);
 
 #ifndef __HAVE_ARCH_STRRCHR
 extern char * strrchr(const char *,int);

lib/string.c

@@ -263,12 +263,12 @@ char * strchr(const char * s, int c)
 }
 #endif
 
-const char *strchrnul(const char *s, int c)
+char *strchrnul(const char *s, int c)
 {
 	for (; *s != (char)c; ++s)
 		if (*s == '\0')
 			break;
-	return s;
+	return (char *)s;
 }
 
 #ifndef __HAVE_ARCH_STRRCHR