BUILD: include sys/socket.h to fix build failure on FreeBSD

Joris Dedieu reported that include/common/standard.h needs this.
diff --git a/include/common/standard.h b/include/common/standard.h
index 511a1dd..93b9e36 100644
--- a/include/common/standard.h
+++ b/include/common/standard.h
@@ -26,6 +26,7 @@
 #include <string.h>
 #include <time.h>
 #include <sys/types.h>
+#include <sys/socket.h>
 #include <sys/un.h>
 #include <netinet/in.h>
 #include <common/config.h>