selftests: pid_namespace: 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/pid_namespace/Makefile

···
 # SPDX-License-Identifier: GPL-2.0
-CFLAGS += -g -I../../../../usr/include/
+CFLAGS += -g $(KHDR_INCLUDES)
 
 TEST_GEN_PROGS = regression_enomem