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>

strndup.h

@@ -0,0 +1,21 @@
+#ifndef STRNDUP_H
+#define STRNDUP_H
+#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
+#endif /* STRNDUP_H */