Use const for constant local variables in wxWidgets headers

This avoids warnings from MSVS 2017 static analyzer when C++ core rule
set is activated.

Closes https://github.com/wxWidgets/wxWidgets/pull/819

include/wx/msw/ole/oleutils.h

@@ -38,7 +38,7 @@
 // return true if ok, false otherwise
 inline bool wxOleInitialize()
 {
-    HRESULT
+    const HRESULT
     hr = ::OleInitialize(NULL);
 
     // RPC_E_CHANGED_MODE indicates that OLE had been already initialized

include/wx/msw/private/fswatcher.h

@@ -207,7 +207,7 @@ public:
         wxCHECK_MSG( m_iocp != INVALID_HANDLE_VALUE, false, "IOCP not init" );
 
         // The special values of 0 will make GetStatus() return Status_Exit.
-        int ret = PostQueuedCompletionStatus(m_iocp, 0, 0, NULL);
+        const int ret = PostQueuedCompletionStatus(m_iocp, 0, 0, NULL);
         if (!ret)
         {
             wxLogSysError(_("Unable to post completion status"));
@@ -244,7 +244,7 @@ public:
         wxCHECK_MSG( count && watch && overlapped, Status_Error,
                      "Output parameters can't be NULL" );
 
-        int ret = GetQueuedCompletionStatus(m_iocp, count, (ULONG_PTR *)watch,
+        const int ret = GetQueuedCompletionStatus(m_iocp, count, (ULONG_PTR *)watch,
                                             overlapped, INFINITE);
         if ( ret != 0 )
         {