Add stub for missing je_malloc_usable_size symbol in geckolib.

ports/geckolib/lib.rs

@@ -39,3 +39,7 @@ mod wrapper;
 pub mod properties {
     include!(concat!(env!("OUT_DIR"), "/properties.rs"));
 }
+
+// FIXME(bholley): This should probably go away once we harmonize the allocators.
+#[no_mangle]
+pub extern "C" fn je_malloc_usable_size(_: *const ::libc::c_void) -> ::libc::size_t { 0 }