xlib: Implement xmesa_get_name by return literal "Mesa " PACKAGE_VERSION directly

This literal is consistence with st_api::name comes from global variable
st_gl_api that will be removed in following commits

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19197>

src/gallium/frontends/glx/xlib/xm_api.c

@@ -914,7 +914,7 @@ void XMesaDestroyVisual( XMesaVisual v )
 const char *
 xmesa_get_name(void)
 {
-   return stapi->name;
+   return "Mesa " PACKAGE_VERSION;
 }