Ensure swift-secp256k1 remains at 0.18.0

Upgrading it caused issues with respect to importing
the package, causing crashes. This should resolve things.
In the future, that package will be upgraded on a case-by-case
basis until the author states it's stable enough to use
without concerns.

Author: MasterJ93

Package.resolved

@@ -20,7 +20,7 @@ let package = Package(
     ],
     dependencies: [
         .package(url: "https://github.com/apple/swift-crypto.git", "1.0.0" ..< "4.0.0"),
-        .package(url: "https://github.com/21-DOT-DEV/swift-secp256k1.git", from: "0.18.0"),
+        .package(url: "https://github.com/21-DOT-DEV/swift-secp256k1.git", exact: "0.18.0"),
         .package(url: "https://github.com/attaswift/BigInt.git", from: "5.5.1")
     ],
     targets: [