samples: rust: fix rust_print build making it a combined module

The `rust_print` module, when built as a module, fails to build with:

    ERROR: modpost: missing MODULE_LICENSE() in samples/rust/rust_print_events.o
    ERROR: modpost: "__tracepoint_rust_sample_loaded" [samples/rust/rust_print.ko] undefined!
    ERROR: modpost: "rust_do_trace_rust_sample_loaded" [samples/rust/rust_print.ko] undefined!

Fix it by building it as a combined one.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Alex Gaynor <alex.gaynor@gmail.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Gary Guo <gary@garyguo.net>
Cc: Björn Roy Baron <bjorn3_gh@protonmail.com>
Cc: Benno Lossin <benno.lossin@proton.me>
Cc: Andreas Hindborg <a.hindborg@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Trevor Gross <tmgross@umich.edu>
Cc: "Linux Next Mailing List" <linux-next@vger.kernel.org>
Link: https://lore.kernel.org/20241111220805.708889-1-ojeda@kernel.org
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/all/20241108152149.28459a72@canb.auug.org.au/
Fixes: 91d3902 ("rust: samples: add tracepoint to Rust sample")
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: ojeda

samples/rust/Makefile

@@ -2,6 +2,8 @@
 ccflags-y += -I$(src)				# needed for trace events
 
 obj-$(CONFIG_SAMPLE_RUST_MINIMAL)		+= rust_minimal.o
-obj-$(CONFIG_SAMPLE_RUST_PRINT)			+= rust_print.o rust_print_events.o
+obj-$(CONFIG_SAMPLE_RUST_PRINT)			+= rust_print.o
+
+rust_print-y := rust_print_main.o rust_print_events.o
 
 subdir-$(CONFIG_SAMPLE_RUST_HOSTPROGS)		+= hostprogs