support/testing: test_iso9660: bump to kernel 4.19.204

support/testing: test_iso9660: bump to kernel 4.19.204

The kernel 4.11 curently used by the test doesn't build with host
gcc >= 10 due the gcc default -fno-common. See GCC 10 porting guide [1].

/usr/bin/ld: scripts/dtc/dtc-parser.tab.o:(.bss+0x20): multiple definition of `yylloc'; scripts/dtc/dtc-lexer.lex.o:(.bss+0x0): first defined here

Update to the latest 4.19 LTS version but doing so requires to
disable CONFIG_RETPOLINE since the toolchain is not retpoline
capable [2].

Fixes:
https://gitlab.com/kubu93/buildroot/-/jobs/1564202078

[1] https://gcc.gnu.org/gcc-10/porting_to.html
[2] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8c4ad5d39144776c2987e81609204e1766ed4190

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>