MAINTAINERS: Add selftests/x86 entry

There are no maintainers specified for tools/testing/selftests/x86.  Shuah has
mentioned [1] that the patches should go through x86 tree or in special cases
directly to Shuah's tree after getting ack-ed from x86 maintainers. Different
people have been confused when sending patches as correct maintainers aren't
found by get_maintainer.pl script. Fix this by adding entry to MAINTAINERS
file.

  [1] https://lore.kernel.org/all/90dc0dfc-4c67-4ea1-b705-0585d6e2ec47@linuxfoundation.org



Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/r/20240610052810.1488793-1-usama.anjum@collabora.com