[klibc] Continue to build on kernels with no pselect system call

We don't want to *require* pselect() just yet.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
diff --git a/klibc/pselect.c b/klibc/pselect.c
index 5266501..f5341c1 100644
--- a/klibc/pselect.c
+++ b/klibc/pselect.c
@@ -20,7 +20,7 @@
 		    timeout, sigmask, sizeof *sigmask);
 }
 
-#else
+#elif defined(__NR_pselect6)
 
 struct __pselect6 {
   const sigset_t *sigmask;