Fix build error on Linux

fix malcommac#66

Author: rinsuki

Sources/Hydra/Promise+Await.swift

@@ -96,7 +96,12 @@ public extension Context {
 	/// - Throws: throw if promise fails
 	@discardableResult
 	internal func await<T>(_ promise: Promise<T>) throws -> T {
-		guard self.queue != DispatchQueue.main else {
+        #if os(Linux)
+        let isNotMainQueue = self.queue.label != DispatchQueue.main.label
+        #else
+        let isNotMainQueue = self.queue != DispatchQueue.main
+        #endif
+		guard isNotMainQueue else {
 			// execute a promise on main context does not make sense
 			// dispatch_semaphore_wait should NOT be called on the main thread.
 			// more here: https://medium.com/@valentinkalchev/how-to-pause-and-resume-a-sequence-of-mutating-swift-structs-using-dispatch-semaphore-fc98eca55c0#.ipbujy4k2