b/t/llvm-efi: define all_files correctly

The llvm-efi toolchain has an incorrect all_files definition.
Bazel's doc says:
> Bazel assumes that all_files is a superset of all other
>artifact-providing attributes ...

Adjust the definition to contain all other depsets (currently just one).

Change-Id: Ic0889698a865355b66927d336067aa028501b97b
Reviewed-on: https://review.monogon.dev/c/monogon/+/2101
Tested-by: Jenkins CI
Reviewed-by: Serge Bazanski <serge@monogon.tech>
diff --git a/build/toolchain/llvm-efi/BUILD.bazel b/build/toolchain/llvm-efi/BUILD.bazel
index 5feb67f..66204db 100644
--- a/build/toolchain/llvm-efi/BUILD.bazel
+++ b/build/toolchain/llvm-efi/BUILD.bazel
@@ -9,9 +9,14 @@
     srcs = ["fltused.o"],
 )
 
+filegroup(
+    name = "all",
+    srcs = [":fltused"],
+)
+
 cc_toolchain(
     name = "efi_k8_cc_toolchain",
-    all_files = ":empty",
+    all_files = ":all",
     ar_files = ":empty",
     as_files = ":empty",
     compiler_files = ":empty",