termscp: fix darwin build

I was running into Foundation.h errors during compilation on mac/darwin,
adding Foundation fixes that for me.

Error:

```
The following warnings were emitted during compilation:

warning: In file included from objc/notify.m:1:
warning: objc/notify.h:1:9: fatal error: 'Foundation/Foundation.h' file not found
warning: #import <Foundation/Foundation.h>
warning: ^~~~~~~~~~~~~~~~~~~~~~~~~
warning: 1 error generated.

error: failed to run custom build command for `mac-notification-sys v0.3.0`

Caused by:
process didn't exit successfully: `/tmp/cargo-installEt3UWW/release/build/mac-notification-sys-f414f8995c05e541/build-script-build` (exit status: 1)
--- stdout
TARGET = Some("x86_64-apple-darwin")
OPT_LEVEL = Some("3")
HOST = Some("x86_64-apple-darwin")
CC_x86_64-apple-darwin = None
CC_x86_64_apple_darwin = None
HOST_CC = None
CC = Some("clang")
CFLAGS_x86_64-apple-darwin = None
CFLAGS_x86_64_apple_darwin = None
HOST_CFLAGS = None
CFLAGS = None
CRATE_CC_NO_DEFAULTS = None
DEBUG = Some("false")
running: "clang" "-O3" "-ffunction-sections" "-fdata-sections" "-fPIC" "--target=x86_64-apple-darwin" "-fmodules" "-o" "/tmp/cargo-installEt3UWW/release/build/mac-notification-sys-ce5dd87fdfef4721/out/objc/notify.o" "-c" "objc/notify.m"
cargo:warning=In file included from objc/notify.m:1:
cargo:warning=objc/notify.h:1:9: fatal error: 'Foundation/Foundation.h' file not found
cargo:warning=#import <Foundation/Foundation.h>
cargo:warning= ^~~~~~~~~~~~~~~~~~~~~~~~~
cargo:warning=1 error generated.
exit status: 1

--- stderr

error occurred: Command "clang" "-O3" "-ffunction-sections" "-fdata-sections" "-fPIC" "--target=x86_64-apple-darwin" "-fmodules" "-o" "/tmp/cargo-installEt3UWW/release/build/mac-notification-sys-ce5dd87fdfef4721/out/objc/notify.o" "-c" "objc/notify.m" with args "clang" did not execute successfully (status code exit status: 1).

warning: build failed, waiting for other jobs to finish...
error: failed to compile `termscp v0.7.0`, intermediate artifacts can be found at `/tmp/cargo-installEt3UWW`

Caused by:
build failed
```

pkgs/tools/networking/termscp/default.nix

···
 , openssl
 , pkg-config
 , rustPlatform
+, Foundation
 , Security
 , stdenv
 }:
···
     libssh
     openssl
   ] ++ lib.optional stdenv.isDarwin [
+    Foundation
     Security
   ];
 

pkgs/top-level/all-packages.nix

···
   telescope = callPackage ../applications/networking/browsers/telescope { };
 
   termscp = callPackage ../tools/networking/termscp {
-    inherit (darwin.apple_sdk.frameworks) Security;
+    inherit (darwin.apple_sdk.frameworks) Foundation Security;
   };
 
   termius = callPackage ../applications/networking/termius { };