gallium: Remove unnecessary forward declaration of swrast_driver_descriptor

MSVC complains that the definition has __declspec(dllexport) but the
forward declaration doesn't. I can't find any users of this decl in
the header.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7680>

src/gallium/include/frontend/sw_driver.h

@@ -16,6 +16,4 @@ struct sw_driver_descriptor
    } winsys[];
 };
 
-extern const struct sw_driver_descriptor swrast_driver_descriptor;
-
 #endif