libstd-rs: Add rustflags RUSTFLAGS="-Zforce-unstable-if-unmarked"

ref: rust-lang/rust#133857

Author: be-neth

recipes-devtools/rust/libstd-rs_1.86.0.bb

@@ -5,3 +5,6 @@ LIC_FILES_CHKSUM = "file://../../COPYRIGHT;md5=11a3899825f4376896e438c8c753f8dc"
 
 # libstd moved from src/libstd to library/std in 1.47+
 S = "${RUSTSRC}/library/std"
+
+# ref: https://github.com/rust-lang/rust/issues/133857
+RUSTFLAGS += "-Zforce-unstable-if-unmarked"