mem: add sane default parameter

This doesn't solve any real problem right now, but it seems
to be the right thing to do. This also is the default for the
types in the `alloc` crate.

Author: phip1611

uefi/src/mem/util.rs

@@ -9,6 +9,8 @@ use core::slice;
 
 #[cfg(not(feature = "unstable"))]
 use ::alloc::alloc::{alloc, dealloc};
+#[cfg(feature = "unstable")]
+use alloc::alloc::Global;
 
 #[cfg(feature = "unstable")]
 use {core::alloc::Allocator, core::ptr::NonNull};
@@ -39,7 +41,7 @@ pub(crate) fn make_boxed<
     // The UEFI data structure.
     Data: Align + ?Sized + Debug + 'a,
     F: FnMut(&'a mut [u8]) -> Result<&'a mut Data, Option<usize>>,
-    #[cfg(feature = "unstable")] A: Allocator,
+    #[cfg(feature = "unstable")] A: Allocator = Global,
 >(
     // A function to read the UEFI data structure into a provided buffer.
     mut fetch_data_fn: F,