Don't rely on pthread_cancel in Fuchsia.

Change-Id: I6eb0870a8e2b04266fc854af345cc5aa09e8a255
diff --git a/glslang/OSDependent/Unix/ossource.cpp b/glslang/OSDependent/Unix/ossource.cpp
index 7e84d4e..0bd5bd9 100644
--- a/glslang/OSDependent/Unix/ossource.cpp
+++ b/glslang/OSDependent/Unix/ossource.cpp
@@ -68,6 +68,9 @@
 // 
 void OS_CleanupThreadData(void)
 {
+#ifdef __Fuchsia__
+	// Fuchsia does not have a meaningful pthread_cancel, so just do nothing.
+#else
 #ifdef __ANDROID__
 	DetachThreadLinux(NULL);
 #else