kconfig: remove const qualifier from sym_expand_string_value()

This function returns realloc'ed memory, so the returned pointer
must be passed to free() when done.  So, 'const' qualifier is odd.
It is allowed to modify the expanded string.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/kconfig/symbol.c

@@ -899,7 +899,7 @@ struct symbol *sym_find(const char *name)
  * name to be expanded shall be prefixed by a '$'. Unknown symbol expands to
  * the empty string.
  */
-const char *sym_expand_string_value(const char *in)
+char *sym_expand_string_value(const char *in)
 {
 	const char *src;
 	char *res;