Ignore GCC warning (-Wclass-memaccess) in imgui.h (#1959)

imgui.h

@@ -52,6 +52,9 @@
 #if defined(__clang__)
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wold-style-cast"
+#elif defined(__GNUC__) && __GNUC__ >= 8
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wclass-memaccess"
 #endif
 
 // Forward declarations
@@ -1922,6 +1925,8 @@ struct ImFont
 
 #if defined(__clang__)
 #pragma clang diagnostic pop
+#elif defined(__GNUC__) && __GNUC__ >= 8
+#pragma GCC diagnostic pop
 #endif
 
 // Include imgui_user.h at the end of imgui.h (convenient for user to only explicitly include vanilla imgui.h)