Fix signed/unsigned comparison warning.

Change-Id: Ibec89dd4463274db4ed7381cf20320a0c9f46a57
Reviewed-on: https://swiftshader-review.googlesource.com/10549
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
diff --git a/src/Common/Thread.hpp b/src/Common/Thread.hpp
index f59a0d4..3b9e674 100644
--- a/src/Common/Thread.hpp
+++ b/src/Common/Thread.hpp
@@ -25,7 +25,7 @@
 	#include <pthread.h>
 	#include <sched.h>
 	#include <unistd.h>
-	#define TLS_OUT_OF_INDEXES (~0)
+	#define TLS_OUT_OF_INDEXES (pthread_key_t)(~0)
 #endif
 
 namespace sw