Make lib imports in overlay explicit

To support the new instrumentation for HWC it would be useful to already
build nOS-V with PAPI support enabled. The enablePapi switch allows it
to be disabled with `nosv.override { enablePapi = false; }`.

Reviewed-by: Aleix Roca Nonell <aleix.rocanonell@bsc.es>
Tested-by: Rodrigo Arias Mallo <rodrigo.arias@bsc.es>

pkgs/nosv/default.nix

@@ -6,6 +6,8 @@
 , pkg-config
 , numactl
 , hwloc
+, papi
+, enablePapi ? true
 , cacheline ? 64 # bits
 , ovni ? null
 , useGit ? false
@@ -56,5 +58,5 @@ in
       numactl
       hwloc
       ovni
-    ];
+    ] ++ lib.optionals enablePapi [ papi ];
   }