make the winapi dependency optional

winapi is only needed in combination with the mmap backend; do
not require it in a no-default-features Windows build.

Signed-off-by: Paolo Bonzini <[email protected]>

Author: bonzini

Cargo.toml

@@ -14,7 +14,7 @@ autobenches = false
 [features]
 default = ["rawfd"]
 backend-bitmap = ["dep:libc"]
-backend-mmap = ["dep:libc"]
+backend-mmap = ["dep:libc", "dep:winapi"]
 backend-atomic = ["arc-swap"]
 rawfd = ["dep:libc"]
 xen = ["backend-mmap", "bitflags", "vmm-sys-util"]
@@ -29,6 +29,7 @@ vmm-sys-util = { version = ">=0.12.1,<=0.14.0", optional = true }
 [target.'cfg(target_family = "windows")'.dependencies.winapi]
 version = "0.3"
 features = ["errhandlingapi", "sysinfoapi"]
+optional = true
 
 [dev-dependencies]
 criterion = "0.5.0"