selftests: rseq: Fix incorrect kernel headers search path

Use $(KHDR_INCLUDES) as lookup path for kernel headers. This prevents
building against kernel headers from the build environment in scenarios
where kernel headers are installed into a specific output directory
(O=...).

Signed-off-by: Mathieu Desnoyers <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Cc: [e-mail]
Cc: Ingo Molnar <[e-mail]>
Cc: <[e-mail]> # 5.18+
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/rseq/Makefile

···
 CLANG_FLAGS += -no-integrated-as
 endif
 
-CFLAGS += -O2 -Wall -g -I./ -I../../../../usr/include/ -L$(OUTPUT) -Wl,-rpath=./ \
+CFLAGS += -O2 -Wall -g -I./ $(KHDR_INCLUDES) -L$(OUTPUT) -Wl,-rpath=./ \
 	  $(CLANG_FLAGS)
 LDLIBS += -lpthread -ldl