fix: add secp256k1 dependency to resolve spm import error (#429)

Package.swift

@@ -14,9 +14,15 @@ let package = Package(
             targets: ["bitchat"]
         ),
     ],
+    dependencies:[
+        .package(url: "https://github.com/21-DOT-DEV/swift-secp256k1", exact: "0.21.1"),
+    ],
     targets: [
         .executableTarget(
             name: "bitchat",
+            dependencies: [
+                .product(name: "P256K", package: "swift-secp256k1")
+            ],
             path: "bitchat",
             exclude: [
                 "Info.plist",
@@ -27,4 +33,4 @@ let package = Package(
             ]
         ),
     ]
-)
+)