[test] add missing WASILibc import in cancellation_handler.swift

Author: kateinoigakukun

test/Concurrency/Runtime/cancellation_handler.swift

@@ -12,6 +12,8 @@
     import Darwin
 #elseif canImport(Glibc)
     import Glibc
+#elseif os(WASI)
+    import WASILibc
 #elseif os(Windows)
     import WinSDK
 #endif