Cargo manifest: disable doctests

In 'cargo test' environments, the SvsmAllocator must not be made the
global_allocator. However, the conditional setting of this attribute depends
on the 'test' configuration predicate being set in 'cargo test' environments.

This seems to not be working properly for documentation tests ([1]), and it's
similar for the alternative 'doctest' predicate ([2]).

As documentation tests currently don't play an important role for the project,
just disable them alltogether until the cargo community has settled on a
suitable alternative.

[1] rust-lang/rust#45599
[2] rust-lang/rust#67295

Signed-off-by: Nicolai Stange <nstange@suse.de>

Cargo.toml

@@ -20,6 +20,7 @@ test = false
 
 [lib]
 test = true
+doctest = false
 
 [dependencies]
 bitflags = "1.3.2"