libyang MAINTENANCE headers includes cleanup

iwyu_tool.py used
diff --git a/src/context.c b/src/context.c
index e8d0eb3..8d074f5 100644
--- a/src/context.c
+++ b/src/context.c
@@ -23,6 +23,7 @@
 
 #include <errno.h>
 #include <pthread.h>
+#include <stdarg.h>
 #include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>