package/rt-tests: needs kernel >= 4.5

rt-tests unconditionally uses CGROUP2_SUPER_MAGIC since
https://git.kernel.org/pub/scm/utils/rt-tests/rt-tests.git/commit/?id=47e857d4bf77440cb59d17c6ed7882a48514f18d
which is only available since kernel 4.5 and
https://github.com/torvalds/linux/commit/67e9c74b8a873408c27ac9a8e4c1d1c8d72c93ff
resulting in the following build failure since bump to version 2.3 in
commit a952b016263cb9ae5f116dac0b3da4ad93a4c55f:

src/sched_deadline/cyclicdeadline.c:381:29: error: 'CGROUP2_SUPER_MAGIC' undeclared (first use in this function)
  ret = mounted(CGROUP_PATH, CGROUP2_SUPER_MAGIC);
                             ^

Fixes:
 - http://autobuild.buildroot.org/results/74766c215a6cc19cd02faec6181ccd5de49c15cf

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

package/rt-tests/Config.in

@@ -1,7 +1,7 @@
 config BR2_PACKAGE_RT_TESTS
 	bool "rt-tests"
 	depends on BR2_TOOLCHAIN_HAS_THREADS_NPTL
-	depends on BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_14 # deadline scheduler syscall
+	depends on BR2_TOOLCHAIN_HEADERS_AT_LEAST_4_5
 	depends on BR2_USE_MMU # fork(), numactl
 	depends on !BR2_STATIC_LIBS # dlopen
 	depends on !BR2_TOOLCHAIN_USES_MUSL # cyclictest
@@ -29,7 +29,7 @@ comment "rt-tests may not work on MIPS with an external uClibc toolchain"
 	depends on BR2_TOOLCHAIN_EXTERNAL_UCLIBC
 	depends on BR2_mips || BR2_mipsel || BR2_mips64 || BR2_mips64el
 
-comment "rt-tests needs a uClibc or glibc toolchain w/ NPTL, headers >= 3.14, dynamic library"
+comment "rt-tests needs a uClibc or glibc toolchain w/ NPTL, headers >= 4.5, dynamic library"
 	depends on !BR2_TOOLCHAIN_HAS_THREADS_NPTL || BR2_STATIC_LIBS \
-		|| BR2_TOOLCHAIN_USES_MUSL || !BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_14
+		|| BR2_TOOLCHAIN_USES_MUSL || !BR2_TOOLCHAIN_HEADERS_AT_LEAST_4_5
 	depends on BR2_USE_MMU