jump_label: rust: pass a mut ptr to static_key_count

When building the rust_print sample with CONFIG_JUMP_LABEL=n, the Rust
static key support falls back to using static_key_count. This function
accepts a mutable pointer to the `struct static_key`, but the Rust
abstractions are incorrectly passing a const pointer.

This means that builds using CONFIG_JUMP_LABEL=n and SAMPLE_RUST_PRINT=y
fail with the following error message:

	error[E0308]: mismatched types
	  --> <root>/samples/rust/rust_print_main.rs:87:5
	   |
	87 | /     kernel::declare_trace! {
	88 | |         /// # Safety
	89 | |         ///
	90 | |         /// Always safe to call.
	91 | |         unsafe fn rust_sample_loaded(magic: c_int);
	92 | |     }
	   | |     ^
	   | |     |
	   | |_____types differ in mutability
	   |       arguments to this function are incorrect
	   |
	   = note: expected raw pointer `*mut kernel::bindings::static_key`
	              found raw pointer `*const kernel::bindings::static_key`
	note: function defined here
	  --> <root>/rust/bindings/bindings_helpers_generated.rs:33:12
	   |
	33 |     pub fn static_key_count(key: *mut static_key) -> c_int;
	   |            ^^^^^^^^^^^^^^^^

To fix this, insert a pointer cast so that the pointer is mutable.

Link: https://lore.kernel.org/20241118202727.73646-1-aliceryhl@google.com
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202411181440.qEdcuyh6-lkp@intel.com/
Fixes: 169484a ("rust: add arch_static_branch")
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

rust/kernel/jump_label.rs

@@ -26,7 +26,7 @@ macro_rules! static_branch_unlikely {
 
         #[cfg(not(CONFIG_JUMP_LABEL))]
         {
-            $crate::bindings::static_key_count(_key) > 0
+            $crate::bindings::static_key_count(_key.cast_mut()) > 0
         }
 
         #[cfg(CONFIG_JUMP_LABEL)]