fix: enable libc dependency under backend-bitmap feature

When enabling the backend-bitmap feature, vm-memory tries to use libc to
determine host page size, however if no other feature that depends on
libc is enabled, this will result in a compilation error

Fixes: 0b7a437 ("make Read/WriteVolatile implementations optional")
Signed-off-by: Patrick Roy <[email protected]>

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"]