Add sys/signal.h for SIGWINCH

--- dvtm.c.orig	Mon Dec 28 16:40:33 2015
+++ dvtm.c	Mon Dec 28 16:57:53 2015
@@ -36,6 +36,11 @@
 #endif
 #include "vt.h"
 
+/* Needed for SIGWINCH */
+#ifdef __OpenBSD__
+#include <sys/signal.h>
+#endif
+
 #ifdef PDCURSES
 int ESCDELAY;
 #endif
@@ -164,8 +169,12 @@ typedef struct {
 } Editor;
 
 #define LENGTH(arr) (sizeof(arr) / sizeof((arr)[0]))
+#ifndef MAX
 #define MAX(x, y)   ((x) > (y) ? (x) : (y))
+#endif
+#ifndef MIN
 #define MIN(x, y)   ((x) < (y) ? (x) : (y))
+#endif
 #define TAGMASK     ((1 << LENGTH(tags)) - 1)
 
 #ifdef NDEBUG