package/bpftool: needs headers >= 4.12

NETLINK_EXT_ACK is only available since kernel 4.12 and
https://github.com/torvalds/linux/commit/2d4bc93368f5a0ddb57c8c885cdad9c9b7a10ed5
resulting in the following build failure since addition of the package
in commit 3675131e6c654d7536a184db0184fde7a97040e0:

net.c: In function 'netlink_open':
net.c:110:36: error: 'NETLINK_EXT_ACK' undeclared (first use in this function); did you mean 'NETLINK_CAP_ACK'?
  110 |  if (setsockopt(sock, SOL_NETLINK, NETLINK_EXT_ACK,
      |                                    ^~~~~~~~~~~~~~~
      |                                    NETLINK_CAP_ACK

Fixes:
 - http://autobuild.buildroot.org/results/c61678e56bd73609838747048f27068818c21150

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/bpftool/Config.in

@@ -5,14 +5,16 @@ config BR2_PACKAGE_BPFTOOL
 	depends on !BR2_STATIC_LIBS # elfutils
 	depends on BR2_TOOLCHAIN_HAS_THREADS # elfutils
 	depends on BR2_TOOLCHAIN_USES_UCLIBC || BR2_TOOLCHAIN_USES_GLIBC # elfutils
+	depends on BR2_TOOLCHAIN_HEADERS_AT_LEAST_4_12
 	select BR2_PACKAGE_BINUTILS
 	select BR2_PACKAGE_ELFUTILS
 	help
 	  bpftool is a tool for for inspection and simple manipulation
 	  of eBPF programs and maps.
 
-comment "bpftool needs a uClibc or glibc toolchain w/ wchar, dynamic library, threads"
+comment "bpftool needs a uClibc or glibc toolchain w/ wchar, dynamic library, threads, headers >= 4.12"
 	depends on !BR2_nios2
 	depends on !BR2_USE_WCHAR || BR2_STATIC_LIBS \
 		|| !BR2_TOOLCHAIN_HAS_THREADS \
-		|| !(BR2_TOOLCHAIN_USES_UCLIBC || BR2_TOOLCHAIN_USES_GLIBC)
+		|| !(BR2_TOOLCHAIN_USES_UCLIBC || BR2_TOOLCHAIN_USES_GLIBC) \
+		|| !BR2_TOOLCHAIN_HEADERS_AT_LEAST_4_12