xen_4_8: fix build w/glibc-2.30

The header (and the `sysctl`-function) were unused, so the inclusion can be
removed safely.

This was done by upstream in 4.9 as well[1].

[1] 74858c9eaf

pkgs/applications/virtualization/xen/4.8.nix

@@ -185,7 +185,10 @@ callPackage (import ./generic.nix (rec {
     # Avoid a glibc >= 2.25 deprecation warnings that get fatal via -Werror.
     sed 1i'#include <sys/sysmacros.h>' \
       -i tools/blktap2/control/tap-ctl-allocate.c \
-      -i tools/libxl/libxl_device.c
+      -i tools/libxl/libxl_device.c \
+      -i tools/qemu-xen/hw/9pfs/9p.c
+
+    sed -i -e '/sys\/sysctl\.h/d' tools/blktap2/drivers/block-remus.c
   '';
 
   passthru.qemu-system-i386 = if withInternalQemu