package/conmon: seccomp needs headers >= 5.0

conmon unconditionally uses seccomp_notif_sizes which is only available
since kernel 5.0 and
https://github.com/torvalds/linux/commit/6a21cc50f0c7f87dae5259f6cfefe024412313f6
resulting in the following build failure since the addition of the
package in commit 06f50ff1bf066feb6cf6ed512113773f87c884fb:

In file included from src/seccomp_notify.h:4,
                 from src/seccomp_notify.c:26:
src/seccomp_notify_plugin.h:33:78: warning: 'struct seccomp_notif_sizes' declared inside parameter list will not be visible outside of this definition or declaration
   33 | typedef int (*run_oci_seccomp_notify_handle_request_cb)(void *opaque, struct seccomp_notif_sizes *sizes, struct seccomp_notif *sreq,
      |                                                                              ^~~~~~~~~~~~~~~~~~~
src/seccomp_notify.c:47:36: error: field 'sizes' has incomplete type
   47 |         struct seccomp_notif_sizes sizes;
      |                                    ^~~~~

Fixes:
 - http://autobuild.buildroot.org/results/cbfa2f3a585b7feecd902137bc589a1ec15cbefe

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/conmon/conmon.mk

@@ -11,7 +11,7 @@ CONMON_LICENSE_FILES = LICENSE
 
 CONMON_DEPENDENCIES = host-pkgconf libglib2
 
-ifeq ($(BR2_PACKAGE_LIBSECCOMP):$(BR2_STATIC_LIBS),y:)
+ifeq ($(BR2_PACKAGE_LIBSECCOMP)$(BR2_TOOLCHAIN_HEADERS_AT_LEAST_5_0):$(BR2_STATIC_LIBS),yy:)
 CONMON_DISABLE_SECCOMP = 0
 CONMON_DEPENDENCIES += libseccomp
 else