Silence bogus GCC 8 -Werror=virtual-move-assign

...seen with at least gcc-c++-8.1.1-1.fc28.x86_64 and
gcc-c++-8.1.1-5.fc28.x86_64 after 5065bcf5
"cppcanvas: avoid -Werror=deprecated-copy (GCC trunk towards GCC 9)" and
6844ef8a "cppcanvas: avoid -Werror=deprecated-
copy (GCC trunk towards GCC 9)":

> [CXX] cppcanvas/source/wrapper/implbitmapcanvas.cxx
> In file included from cppcanvas/source/wrapper/implbitmapcanvas.hxx:26,
>                  from cppcanvas/source/wrapper/implbitmapcanvas.cxx:28:
> include/cppcanvas/bitmapcanvas.hxx:41:11: error: defaulted move assignment for ‘cppcanvas::BitmapCanvas’ calls a non-trivial move assignment operator for virtual base ‘cppcanvas::Canvas’ [-Werror=virtual-move-assign]
>      class BitmapCanvas : public virtual Canvas
>            ^~~~~~~~~~~~

Change-Id: I5348eb110c147957347554504a7e7c6d7fb32b70
Reviewed-on: https://gerrit.libreoffice.org/58185
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

include/cppcanvas/canvas.hxx

@@ -73,7 +73,9 @@ namespace cppcanvas
         Canvas(Canvas const &) = default;
         Canvas(Canvas &&) = default;
         Canvas & operator =(Canvas const &) = default;
+#if !(defined __GNUC__ && !defined __clang__ && __GNUC__ == 8) // bogus -Werror=virtual-move-assign
         Canvas & operator =(Canvas &&) = default;
+#endif
 
         virtual ~Canvas() {}