macaddr: add #include unistd.h for close() function

this fixes a compile warning
diff --git a/src/macaddr.c b/src/macaddr.c
index 8b78cb6..392ee13 100644
--- a/src/macaddr.c
+++ b/src/macaddr.c
@@ -1,6 +1,7 @@
 #include <stdint.h>
 #include <string.h>
 #include <stdlib.h>
+#include <unistd.h>
 
 
 int osmo_macaddr_parse(uint8_t *out, const char *in)