lkl: Let `mount` find `lklfuse` mount helper

This enables mount to find lklfuse when mounting with the type
"fuse.lklfuse". With this fix, lklfuse can be used to mount filesystems
with NixOS filesystem options.

This is the same fix as has been applied to bindfs, mergerfs, and
apfs-fuse.

pkgs/applications/virtualization/lkl/default.nix

···
        tools/lkl/lib/hijack/liblkl-hijack.so $lib/lib
   '';
 
+  postFixup = ''
+    ln -s $out/bin/lklfuse $out/bin/mount.fuse.lklfuse
+  '';
+
   # We turn off format and fortify because of these errors (fortify implies -O2, which breaks the jitter entropy code):
   #   fs/xfs/xfs_log_recover.c:2575:3: error: format not a string literal and no format arguments [-Werror=format-security]
   #   crypto/jitterentropy.c:54:3: error: #error "The CPU Jitter random number generator must not be compiled with optimizations. See documentation. Use the compiler switch -O0 for compiling jitterentropy.c."