libnatpmp: skip version check

This fixes:
```
natpmpc: [skip] Version check override
natpmpc: No executables in the package provided version 20230423
natpmpc: Generic tests failed
```
Ref: https://github.com/openwrt/packages/actions/runs/37582454054/job/112685069909?pr=30718

Signed-off-by: Andy Chiang <AndyChiang_git@outlook.com>

libs/libnatpmp/test-version.sh

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+case "$1" in
+libnatpmp1|natpmpc)
+	exit 0
+	;;
+*)
+	echo "test-version.sh: unhandled sub-package '$1'" >&2
+	exit 1
+	;;
+esac