selftests: fix header dependency for pid_namespace selftests

The way the test target was defined before, when building with clang we
get a command line like this:

clang -Wall -Werror -g -I../../../../usr/include/ \
	regression_enomem.c ../pidfd/pidfd.h  -o regression_enomem

This yields an error, because clang thinks we want to produce both a *.o
file, as well as a precompiled header:

clang: error: cannot specify -o when generating multiple output files

gcc, for whatever reason, doesn't exhibit the same behavior which I
suspect is why the problem wasn't noticed before.

This can be fixed simply by using the LOCAL_HDRS infrastructure the
selftests lib.mk provides. It does the right think and marks the target
as depending on the header (so if the header changes, we rebuild), but
it filters the header out of the compiler command line, so we don't get
the error described above.
Signed-off-by: NAxel Rasmussen <axelrasmussen@google.com>
Reviewed-by: NChristian Brauner <brauner@kernel.org>
Signed-off-by: NShuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/pid_namespace/Makefile

 # SPDX-License-Identifier: GPL-2.0
 CFLAGS += -g -I../../../../usr/include/
 
-TEST_GEN_PROGS := regression_enomem
+TEST_GEN_PROGS = regression_enomem
 
-include ../lib.mk
+LOCAL_HDRS += $(selfdir)/pidfd/pidfd.h
 
-$(OUTPUT)/regression_enomem: regression_enomem.c ../pidfd/pidfd.h
+include ../lib.mk