(#17) Added extern declaration to avoid symbol mangeling

Author: s1hofmann

Diff for: src/linux/window_manager.cc

@@ -1,7 +1,9 @@
 #include <X11/Xlib.h>
 #include <X11/Xutil.h>
 #include "../window_manager.h"
-#include "../xdisplay.h"
+extern "C" {
+    #include "../xdisplay.h"
+}
 
 WindowHandle getActiveWindow() {
     Display* xServer = XGetMainDisplay();