assets/nyxt.desktop: Fix missing icon on Wayland
assets/nyxt.desktop: Fix missing icon on Wayland

(gdk:gdk-set-program-class "nyxt") sets the window manager class (WM
class) to "nyxt" under the X window system, whereas on Wayland this has
no effect so the WM class is "sbcl" (the default).

Fixes issue #1107.

jmercouris/NeXT,jmercouris/NeXT