add missing truffle boundary

Author: timfel

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/PosixModuleBuiltins.java

@@ -1743,6 +1743,7 @@ boolean access(String path, int mode, Object dirFd, boolean effectiveIds, boolea
     @Builtin(name = "cpu_count", minNumOfPositionalArgs = 0)
     @GenerateNodeFactory
     abstract static class CpuCountNode extends PythonBuiltinNode {
+        @TruffleBoundary
         @Specialization
         static int getCpuCount() {
             return Runtime.getRuntime().availableProcessors();