rust_out_of_tree: use GPL as the license string instead of GPL v2

See commit dda49a0a5bb6 ("treewide: use `GPL` as the license string
instead of `GPL v2`") in the main repository:

    Since commit bf7fbeeae6db ("module: Cure the MODULE_LICENSE "GPL" vs.
    "GPL v2" bogosity"), Documentation/process/license-rules.rst explains
    that `MODULE_LICENSE` arguments can simply use `GPL` instead of `GPL v2`,
    because it is not meant to replace the proper licensing information.

    Since the `license:` field of the `module!` macro is the same as
    `MODULE_LICENSE`, use the simpler form in Rust too.

Link: Rust-for-Linux/linux@dda49a0
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

rust_out_of_tree.rs

@@ -9,7 +9,7 @@ module! {
     name: b"rust_out_of_tree",
     author: b"Rust for Linux Contributors",
     description: b"Rust out-of-tree sample",
-    license: b"GPL v2",
+    license: b"GPL",
 }
 
 struct RustOutOfTree {