fix: enable libc dependency with backend-bitmap feature

backend-bitmap uses libc to determine host page size in
atomic_bitmap.rs. Currently, commands such as cargo build
--no-default-features --features backend-bitmap fail, because libc is an
optional dependency, and only enabled if we also enable backend-mmap or
rawfd (which there is no reasonable setup in which this wasnt the case,
explaining why no one ever ran into this).

Technically the libc dependency is only needed on target_family =
"unix", but there's no way to enable a feature only on a specific
target, and on non-unix systems the libc call is disabled via cfg-ery,
so won't cause any problems.

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

Author: roypat

Cargo.toml

@@ -13,7 +13,7 @@ autobenches = false
 
 [features]
 default = ["rawfd"]
-backend-bitmap = []
+backend-bitmap = ["dep:libc"]
 backend-mmap = ["dep:libc"]
 backend-atomic = ["arc-swap"]
 rawfd = ["dep:libc"]