Add strndup.h header so we can use this useful function

It's missing on Windows... we had this helper in liquivision.c but since I
used the function in git-access.c I figured I should just turn it into a
little helper.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

liquivision.c

@@ -3,7 +3,7 @@
 #include "dive.h"
 #include "divelist.h"
 #include "file.h"
-
+#include "strndup.h"
 
 // Convert bytes into an INT
 #define array_uint16_le(p) ((unsigned int) (p)[0] \
@@ -12,21 +12,6 @@
 							+ ((p)[1]<<8) + ((p)[2]<<16) \
 							+ ((p)[3]<<24))
 
-#if __WIN32__
-static char *strndup (const char *s, size_t n)
-{
-	char *cpy;
-	size_t len = strlen(s);
-	if (n < len)
-		len = n;
-	if ((cpy = malloc(len + 1)) != NULL) {
-		cpy[len] = '\0';
-		memcpy(cpy, s, len);
-	}
-	return cpy;
-}
-#endif
-
 struct lv_event {
 	time_t time;
 	struct pressure {