Dependency on heapless version isn't optional

[jsprog]

The core has optional support for heapless@0.7 and heapless@0.8, but not littlefs2

# https://github.com/trussed-dev/littlefs2/blob/main/Cargo.toml

[dependencies]
bitflags = "2.9.0"
delog = "0.1.0"
generic-array = "0.14"
heapless = "0.7"
littlefs2-core = { version = "0.1", path = "core" }
littlefs2-sys = { version = "0.3.1", features = ["multiversion"] }