Suppress bogus -Wnoexcept from gcc9 in wx/graphics.h

Due to what looks like a bug, gcc 9.3.0 gives the following incomplete
error message without it:

include/wx/graphics.h:278:7: error: but
‘wxGraphicsGradientStop::wxGraphicsGradientStop(wxGraphicsGradientStop&&)’
does not throw; perhaps it should be declared ‘noexcept’
[-Werror=noexcept]

(without any other diagnostics).

include/wx/graphics.h

@@ -274,6 +274,12 @@ extern WXDLLIMPEXP_DATA_CORE(wxGraphicsMatrix) wxNullGraphicsMatrix;
 // and how they are spread out in a gradient
 // ----------------------------------------------------------------------------
 
+// gcc 9 gives a nonsensical warning about implicitly generated move ctor not
+// throwing but not being noexcept, suppress it.
+#if wxCHECK_GCC_VERSION(9, 1) && !wxCHECK_GCC_VERSION(10, 0)
+wxGCC_WARNING_SUPPRESS(noexcept)
+#endif
+
 // Describes a single gradient stop.
 class wxGraphicsGradientStop
 {
@@ -306,6 +312,10 @@ private:
     float m_pos;
 };
 
+#if wxCHECK_GCC_VERSION(9, 1) && !wxCHECK_GCC_VERSION(10, 0)
+wxGCC_WARNING_RESTORE(noexcept)
+#endif
+
 // A collection of gradient stops ordered by their positions (from lowest to
 // highest). The first stop (index 0, position 0.0) is always the starting
 // colour and the last one (index GetCount() - 1, position 1.0) is the end