liballoc: alloc-extern-crates test needs --edition=2018

Author: Centril

src/test/run-make-fulldeps/alloc-extern-crates/Makefile

@@ -2,4 +2,4 @@
 
 all:
 	$(RUSTC) fakealloc.rs
-	$(RUSTC) --crate-type=rlib ../../../liballoc/lib.rs --cfg feature=\"external_crate\" --extern external=$(TMPDIR)/$(shell $(RUSTC) --print file-names fakealloc.rs)
+	$(RUSTC) --edition=2018 --crate-type=rlib ../../../liballoc/lib.rs --cfg feature=\"external_crate\" --extern external=$(TMPDIR)/$(shell $(RUSTC) --print file-names fakealloc.rs)