rust-hello-world: enable LTO in the build

If the libstd-rs recipe is not correctly embedding LVM bitcode in its
output, then it can produce a toolchain that appears to work until you
try building a project that requests LTO:

    error: failed to get bitcode from object file for LTO (Bitcode section not found in object file)

Enable LTO for rust-hello-world to give us an easy watch to catch such
problems.

recipes-example/rust-hello-world/rust-hello-world/0001-enable-LTO.patch

@@ -0,0 +1,23 @@
+From fa40b874f6470ec11a8fd7b0c9909d0cdd2d6feb Mon Sep 17 00:00:00 2001
+From: Dan Callaghan <dan.callaghan@opengear.com>
+Date: Fri, 5 Feb 2021 08:56:34 +1000
+Subject: [PATCH] enable LTO
+
+---
+ Cargo.toml | 3 +++
+ 1 file changed, 3 insertions(+)
+
+diff --git a/Cargo.toml b/Cargo.toml
+index 7a2f6c8..cdb6b5d 100644
+--- a/Cargo.toml
++++ b/Cargo.toml
+@@ -3,3 +3,6 @@
+ name = "rust-hello-world"
+ version = "0.0.1"
+ authors = ["Cody P Schafer <dev@codyps.com>"]
++
++[profile.release]
++lto = true
+-- 
+2.28.0
+

recipes-example/rust-hello-world/rust-hello-world_git.bb

@@ -4,6 +4,10 @@ SRC_URI = "git://github.com/jmesmon/rust-hello-world.git;protocol=https"
 SRCREV="e0fa23f1a3cb1eb1407165bd2fc36d2f6e6ad728"
 LIC_FILES_CHKSUM="file://COPYRIGHT;md5=e6b2207ac3740d2d01141c49208c2147"
 
+SRC_URI += "\
+    file://0001-enable-LTO.patch \
+    "
+
 SUMMARY = "Hello World by Cargo for Rust"
 HOMEPAGE = "https://github.com/jmesmon/rust-hello-world"
 LICENSE = "MIT | Apache-2.0"