Define soHandle when on Linux

Much like hModule is defined when on Windows, it is necessary to define
VSTGUI::soHandle when on Linux.  This keeps loading the plugin from
raising an undefined symbol error.

Fixes: surge-synthesizer#38

src/vst2/Vst2PluginInstance.cpp

@@ -58,6 +58,8 @@ BOOL WINAPI DllMain(HINSTANCE hInst, DWORD dwReason, LPVOID lpvReserved)
    return 1;
 }
 } // extern "C"
+#elif __linux__
+namespace VSTGUI { void* soHandle = nullptr; }
 #endif
 
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