Fix uclibc build issue

This patch fixes the following compilation error raised by the bump to
version 1.13.1 in Buildroot [1] using uclibc as the C library for the
cross-compilation toolchain:

net.c: In function 'babel_send':
net.c:199:27: error: 'IPV6_DONTFRAG' undeclared (first use in this function)
  199 |         cmsg->cmsg_type = IPV6_DONTFRAG;;

[1] https://patchwork.ozlabs.org/project/buildroot/patch/[email protected]/
Signed-off-by: Dario Binacchi <[email protected]>

net.c

@@ -37,6 +37,10 @@ THE SOFTWARE.
 #include <arpa/inet.h>
 #include <errno.h>
 
+#if defined(__UCLIBC__)
+#include <linux/in6.h>
+#endif
+
 #include "babeld.h"
 #include "util.h"
 #include "net.h"