Kconfig 5.8 (since commit 09d5873e4d1f70202314b5fe40160f9b14b9d2d0)
blocks using the source statement within choice but that's a pattern we
use intensively. Re-enable it.

Index: kconfig/parser.y
===================================================================
--- kconfig.orig/parser.y
+++ kconfig/parser.y
@@ -134,6 +134,7 @@ stmt_list_in_choice:
 	| stmt_list_in_choice comment_stmt
 	| stmt_list_in_choice config_stmt
 	| stmt_list_in_choice if_stmt_in_choice
+	| stmt_list_in_choice source_stmt
 	| stmt_list_in_choice error T_EOL	{ zconf_error("invalid statement"); }
 ;