CMakeLists.txt: use visibility=hidden by default, so that unneeded libtownengine.so symbols are not leaked

include/twn_engine_api.h

@@ -4,7 +4,7 @@
 #if defined(__WIN32)
 	#define TWN_API __declspec(dllexport)
 #else
-	#define TWN_API
+	#define TWN_API __attribute__((visibility("default")))
 #endif
 
 #endif