kconfig: Make expr_copy() take a const argument

Fixes
scripts/kconfig/expr.c: In function ‘expr_get_leftmost_symbol’:
scripts/kconfig/expr.c:1026:2: warning: passing argument 1 of ‘expr_copy’ discards qualifiers from pointer target type
scripts/kconfig/expr.c:67:14: note: expected ‘struct expr *’ but argument is of type ‘const struct expr *’

Signed-off-by: Michal Marek <mmarek@suse.cz>
diff --git a/scripts/kconfig/expr.c b/scripts/kconfig/expr.c
index 65531a7..0010034 100644
--- a/scripts/kconfig/expr.c
+++ b/scripts/kconfig/expr.c
@@ -64,7 +64,7 @@
 	return e2 ? expr_alloc_two(E_OR, e1, e2) : e1;
 }
 
-struct expr *expr_copy(struct expr *org)
+struct expr *expr_copy(const struct expr *org)
 {
 	struct expr *e;