imap-send: Add missing #include for macosx

There is a compile error without that.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

imap-send.c

@@ -28,6 +28,7 @@
 #include <netinet/in.h>
 #include <netinet/tcp.h>
 #include <arpa/inet.h>
+#include <sys/socket.h>
 #include <netdb.h>
 
 typedef struct store_conf {