Make wxImage ctor from XPM data explicit

This avoids accidental conversions from XPM to wxImage which can result
in the use of an expected wxBitmap ctor later.

See #19149.

include/wx/image.h

@@ -310,7 +310,7 @@ public:
         { LoadFile( name, type, index ); }
     wxImage( const wxString& name, const wxString& mimetype, int index = -1 )
         { LoadFile( name, mimetype, index ); }
-    wxImage( const char* const* xpmData )
+    explicit wxImage( const char* const* xpmData )
         { Create(xpmData); }
 
 #if wxUSE_STREAMS

interface/wx/image.h

@@ -611,8 +611,10 @@ public:
         @beginWxPerlOnly
         Not supported by wxPerl.
         @endWxPerlOnly
+
+        This constructor has become @c explicit in wxWidgets 3.1.6.
     */
-    wxImage(const char* const* xpmData);
+    explicit wxImage(const char* const* xpmData);
 
     /**
         Creates an image from a file.