um: Add missing NR_CPUS include

We need linux/threads.h for that variable.

Fixes: 8bba077066d6d0 ("um: Set number of CPUs")
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/os-Linux/skas/process.c

@@ -21,6 +21,7 @@
 #include <registers.h>
 #include <skas.h>
 #include <sysdep/stub.h>
+#include <linux/threads.h>
 
 int is_skas_winch(int pid, int fd, void *data)
 {