Fix issues from review

- Use lowercase win32 header names for cross-compiling with MinGW
- Correct a unix GUID forward declaration

Sources/Plasma/Apps/plClient/winmain.cpp

@@ -46,7 +46,7 @@ You can contact Cyan Worlds, Inc. by email legal@cyan.com
 #include <stdio.h>
 #include <direct.h>     // windows directory handling fxns (for chdir)
 #include <process.h>
-#include <Shellapi.h>   // ShellExecuteA
+#include <shellapi.h>   // ShellExecuteA
 
 //#define DETACH_EXE  // Microsoft trick to force loading of exe to memory 
 #ifdef DETACH_EXE

Sources/Plasma/CoreLib/hsWindows.h

@@ -63,7 +63,7 @@ You can contact Cyan Worlds, Inc. by email legal@cyan.com
 
 #   define NOMINMAX
 #   define WIN32_LEAN_AND_MEAN
-#   include <Windows.h>
+#   include <windows.h>
 #   include <ws2tcpip.h> // Pulls in WinSock 2 for us
 
     // This needs to be after #include <windows.h>, since it also includes windows.h

Sources/Plasma/PubUtilLib/plAudio/plEAXEffects.h

@@ -68,7 +68,7 @@ typedef struct _EAXREVERBPROPERTIES EAXREVERBPROPERTIES;
 #if HS_BUILD_FOR_WIN32
     typedef struct _GUID GUID;
 #else
-#    define GUID char*
+    typedef char* GUID;
 #endif
 
 class plEAXListener