Update threadpool.cpp

Author: GregoryComer

extension/threadpool/threadpool.cpp

@@ -127,7 +127,7 @@ ThreadPool* get_threadpool() {
 #else
     // Set threads equal to the number of performance cores.
     auto result =
-        ::executorch::extension::cpuinfo::get_num_performant_cores();
+        static_cast<int>(::executorch::extension::cpuinfo::get_num_performant_cores());
 #endif
 
     /*