Switch from stdcall to ccall on Windows

Otherwise GHC compliains:

C:\projects\threadscope\GUI\GtkExtras.hs:94:1: warning: [-Wunsupported-calling-conventions]
    * the 'stdcall' calling convention is unsupported on this platform,
      treating as ccall
    * When checking declaration:
        foreign import stdcall unsafe "static shlobj.h ShellExecuteA" c_ShellExecuteA
          :: Ptr ()
             -> CString -> CString -> CString -> CString -> CInt -> IO CInt

Author: Mitsutoshi Aoe

GUI/GtkExtras.hs

@@ -94,7 +94,7 @@ launchProgramForURI uri = do
             1       -- SW_SHOWNORMAL
     return True
 
-foreign import stdcall unsafe "shlobj.h ShellExecuteA"
+foreign import ccall unsafe "shlobj.h ShellExecuteA"
     c_ShellExecuteA :: Ptr ()  -- HWND hwnd
                     -> CString -- LPCTSTR lpOperation
                     -> CString -- LPCTSTR lpFile