Fix alloc_jemalloc debug feature

At least, I think that's how it should be.  'debug' is how the feature is called in Cargo.toml.

src/liballoc_jemalloc/build.rs

@@ -111,7 +111,7 @@ fn main() {
         cmd.arg("--with-jemalloc-prefix=je_");
     }
 
-    if cfg!(feature = "debug-jemalloc") {
+    if cfg!(feature = "debug") {
         cmd.arg("--enable-debug");
     }