libyang MAINTENANCE headers includes cleanup

iwyu_tool.py used
diff --git a/src/schema_compile.c b/src/schema_compile.c
index 19e8a3a..ab376ec 100644
--- a/src/schema_compile.c
+++ b/src/schema_compile.c
@@ -31,6 +31,7 @@
 #include "log.h"
 #include "parser_schema.h"
 #include "path.h"
+#include "plugins.h"
 #include "plugins_exts.h"
 #include "plugins_exts_compile.h"
 #include "plugins_internal.h"