linux-user: Fix compilation when F_SETPIPE_SZ isn't defined

Older kernels don't have F_SETPIPE_SZ and F_GETPIPE_SZ (in
particular RHEL6's system headers don't define these). Add
ifdefs so that we can gracefully fall back to not supporting
those guest ioctls rather than failing to build.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-id: 1467304429-21470-1-git-send-email-peter.maydell@linaro.org

linux-user/syscall.c

@@ -5598,10 +5598,12 @@ static int target_to_host_fcntl_cmd(int cmd)
 	case TARGET_F_SETOWN_EX:
 	    return F_SETOWN_EX;
 #endif
+#ifdef F_SETPIPE_SZ
         case TARGET_F_SETPIPE_SZ:
             return F_SETPIPE_SZ;
         case TARGET_F_GETPIPE_SZ:
             return F_GETPIPE_SZ;
+#endif
 	default:
             return -TARGET_EINVAL;
     }