Make wxTemp[F]File classes non default ctor explicit

There should really be no reason to ever implicitly convert a string to
a file.

interface/wx/ffile.h

@@ -54,7 +54,7 @@ public:
         @warning
         You should use IsOpened() to verify that the constructor succeeded.
     */
-    wxTempFFile(const wxString& strName);
+    explicit wxTempFFile(const wxString& strName);
 
     /**
         Destructor calls Discard() if temporary file is still open.

interface/wx/file.h

@@ -52,7 +52,7 @@ public:
         @warning
         You should use IsOpened() to verify that the constructor succeeded.
     */
-    wxTempFile(const wxString& strName);
+    explicit wxTempFile(const wxString& strName);
 
     /**
         Destructor calls Discard() if temporary file is still open.