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: e6fbd17 ("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

Author: Bala-Vignesh-Reddy

tools/testing/selftests/x86/Makefile

@@ -36,6 +36,7 @@ BINARIES_32 := $(patsubst %,$(OUTPUT)/%,$(BINARIES_32))
 BINARIES_64 := $(patsubst %,$(OUTPUT)/%,$(BINARIES_64))
 
 CFLAGS := -O2 -g -std=gnu99 -pthread -Wall $(KHDR_INCLUDES)
+CFLAGS += -I $(top_srcdir)/tools/testing/selftests/
 
 # call32_from_64 in thunks.S uses absolute addresses.
 ifeq ($(CAN_BUILD_WITH_NOPIE),1)