Avoid including DFHackVersion.h in PluginManager.h

Changes to DFHackVersion.h previously required recompiling nearly all
of the DFHack core.

library/Core.cpp

@@ -1066,7 +1066,9 @@ void fIOthread(void * iodata)
     run_dfhack_init(con, core);
 
     con.print("DFHack is ready. Have a nice day!\n"
-              "Type in '?' or 'help' for general help, 'ls' to see all commands.\n");
+              "DFHack version %s\n"
+              "Type in '?' or 'help' for general help, 'ls' to see all commands.\n",
+              Version::dfhack_version());
 
     int clueless_counter = 0;
     while (true)

library/include/DFHackVersion.h

@@ -1,3 +1,4 @@
+#pragma once
 namespace DFHack {
     namespace Version {
         const char *dfhack_version();

library/include/PluginManager.h

@@ -24,7 +24,6 @@ distribution.
 
 #pragma once
 
-#include "DFHackVersion.h"
 #include "Export.h"
 #include "Hooks.h"
 #include "ColorText.h"
@@ -59,6 +58,10 @@ namespace DFHack
         class Notification;
     }
 
+    namespace Version {
+        const char *dfhack_version();
+    }
+
     // anon type, pretty much
     struct DFLibrary;