zlib.h for Z_BEST_COMPRESSION

Index: rts/Sim/Path/Default/PathEstimator.cpp
--- rts/Sim/Path/Default/PathEstimator.cpp.orig
+++ rts/Sim/Path/Default/PathEstimator.cpp
@@ -30,6 +30,10 @@
 #include "System/StringUtil.h"
 #include "System/Sync/SHA512.hpp"
 
+#ifdef __OpenBSD__
+	#include <zlib.h>
+#endif
+
 #define ENABLE_NETLOG_CHECKSUM 1
 
 CONFIG(int, PathingThreadCount).defaultValue(0).safemodeValue(1).minimumValue(0);
