aconfig: fix workspace resolver warning

Fix the following warning:

  warning: virtual workspace defaulting to `resolver = "1"` despite one or more workspace members being on edition 2021 which implies `resolver = "2"`
  note: to keep the current resolver, specify `workspace.resolver = "1"` in the workspace root's manifest
  note: to use the edition 2021 resolver, specify `workspace.resolver = "2"` in the workspace root's manifest
  note: for more details see https://doc.rust-lang.org/cargo/reference/resolver.html#resolver-versions

Further info: https://doc.rust-lang.org/edition-guide/rust-2021/default-cargo-resolver.html

Bug: N/A
Test: cargo c # no warnings related to the resolver
Change-Id: I5ee546e4f2ba9e854026bf4baee380e89cf4a01b
diff --git a/tools/aconfig/Cargo.toml b/tools/aconfig/Cargo.toml
index 535ff21..970fdcf 100644
--- a/tools/aconfig/Cargo.toml
+++ b/tools/aconfig/Cargo.toml
@@ -6,3 +6,5 @@
     "aconfig_storage_file",
     "printflags"
 ]
+
+resolver = "2"