lib/nl: add missing header

CMSG_NXTHDR requires <linux/socket.h>. This fix a build error with the musl
C library:

```
undefined reference to `__cmsg_nxthdr'
```

https://github.com/thom311/libnl/pull/83

1 files changed, 1 insertions, 0 deletions

diff --git a/lib/nl.c b/lib/nl.c
index f06e9a0..1e1b1ed 100644
--- a/lib/nl.c
+++ b/lib/nl.c
@@ -32,6 +32,7 @@
 #include <netlink/handlers.h>
 #include <netlink/msg.h>
 #include <netlink/attr.h>
+#include <linux/socket.h>
 
 /**
  * @defgroup core_types Data Types