imap-send: fix compilation-error on Windows

mmsystem.h (included from windows.h) defines DRV_OK to 1. To avoid
an error due to DRV_OK redefenition, this patch undefines the old
definition (i.e the one from mmsystem.h) before defining DRV_OK.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

imap-send.c

@@ -94,6 +94,7 @@ struct msg_data {
 	unsigned int crlf:1;
 };
 
+#undef DRV_OK
 #define DRV_OK          0
 #define DRV_MSG_BAD     -1
 #define DRV_BOX_BAD     -2