selftests/x86: Add selftests include path for kselftest.h after centralization

selftests/x86: Add selftests include path for kselftest.h after centralization

The previous change centralizing kselftest.h include path in lib.mk caused x86
selftests to fail, as x86 Makefile overwrites CFLAGS using ":=", dropping the
include path added in lib.mk. Therefore, helpers.h could not find kselftest.h
during compilation.

Fix this by adding the tools/testing/sefltest to CFLAGS in x86 Makefile.

  [ bp: Correct commit ID in Fixes: ]

Fixes: e6fbd1759c9e ("selftests: complete kselftest include centralization")
Closes: https://lore.kernel.org/lkml/CA+G9fYvKjQcCBMfXA-z2YuL2L+3Qd-pJjEUDX8PDdz2-EEQd=Q@mail.gmail.com/T/#m83fd330231287fc9d6c921155bee16c591db7360
Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Signed-off-by: Bala-Vignesh-Reddy <reddybalavignesh9979@gmail.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Tested-by: Anders Roxell <anders.roxell@linaro.org>
Tested-by: Brendan Jackman <jackmanb@google.com>
Link: https://patch.msgid.link/20251022062948.162852-1-reddybalavignesh9979@gmail.com