libpsm2: disable blanket -Werror (gcc-11 fix)

Without the change gcc-11 build fails as:

```
$ nix-build -E 'with import ./. {}; libpsm2.override { stdenv = gcc11Stdenv; }
...
/build/source/include/linux-x86_64/sysdep.h:146:9:
  error: array subscript 'struct xchg_dummy[0]'
    is partly outside array bounds of 'psmi_lock_t[1]'
```

pkgs/os-specific/linux/libpsm2/default.nix

@@ -13,6 +13,12 @@ stdenv.mkDerivation rec {
 
   buildInputs = [ numactl pkg-config ];
 
+  makeFlags = [
+    # Disable blanket -Werror to avoid build failures
+    # on fresh toolchains like gcc-11.
+    "WERROR="
+  ];
+
   installFlags = [
     "DESTDIR=$(out)"
     "UDEVDIR=/etc/udev"