test: add missing import WASILibc to cancellation_handler.swift (#67143)

This allows this test to build and run when targeting WASI.

Author: MaxDesiatov

test/Concurrency/Runtime/cancellation_handler.swift

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