fix libosmocore build for NuttX target

this fixes some compilation issues with libosmocore under NuttX,
particularly as some #defines are missing or some header files are
slightly different.
diff --git a/src/macaddr.c b/src/macaddr.c
index 7bc4401..59c481f 100644
--- a/src/macaddr.c
+++ b/src/macaddr.c
@@ -89,6 +89,7 @@
 
 #include <sys/ioctl.h>
 #include <net/if.h>
+#include <netinet/in.h>
 #include <netinet/ip.h>
 
 int osmo_get_macaddr(uint8_t *mac_out, const char *dev_name)