Fix build on all platforms (#185)

Motivation:

It turns out that #183 broke the build on a number of platforms,
including Linux 6.1+, Android 6.1+, and on 6.0 macOS. We need to keep
things building.

Modifications:

Guard the import of CDispatch
Remove the use of replacingOccurrences

Result:

Better builds.

Sources/NIOSSH/Keys And Signatures/NIOSSHCertifiedPublicKey.swift

@@ -12,11 +12,14 @@
 //
 //===----------------------------------------------------------------------===//
 
-import CDispatch
 import Crypto
 import Dispatch
 import NIOCore
 
+#if canImport(CDispatch)
+import CDispatch
+#endif
+
 #if canImport(FoundationEssentials)
 import FoundationEssentials
 #else
@@ -393,22 +396,13 @@ extension NIOSSHCertifiedPublicKey: Hashable {}
 
 extension NIOSSHCertifiedPublicKey: CustomDebugStringConvertible {
     public var debugDescription: String {
-        """
-        NIOSSHCertifiedPublicKey(
-            nonce: \(self.nonce),
-            serial: \(self.serial),
-            type: \(self.type),
-            key: \(self.key),
-            keyID: \(self.keyID),
-            validPrincipals: \(self.validPrincipals),
-            validAfter: \(self.validAfter),
-            validBefore: \(self.validBefore),
-            criticalOptions: \(self.criticalOptions),
-            extensions: \(self.extensions),
-            signatureKey: \(self.signatureKey),
-            signature: \(self.signature)
-        )
-        """.replacingOccurrences(of: "\n", with: "")
+        // Slightly hacky multiline string to try to keep things
+        // clear.
+        "NIOSSHCertifiedPublicKey(nonce: \(self.nonce), serial: \(self.serial), type: \(self.type), "
+            + "key: \(self.key), keyID: \(self.keyID), validPrincipals: \(self.validPrincipals), "
+            + "validAfter: \(self.validAfter), validBefore: \(self.validBefore), "
+            + "criticalOptions: \(self.criticalOptions), extensions: \(self.extensions), "
+            + "signatureKey: \(self.signatureKey), signature: \(self.signature))"
     }
 }