selftests: Fix seccomp to support relocatable build (O=objdir)

Fix seccomp relocatable builds. This is a simple fix to use the
right lib.mk variable TEST_GEN_PROGS. Local header dependency
is addressed in a change to lib.mk as a framework change that
enforces the dependency without requiring changes to individual
tests.

The following use-cases work with this change:

In seccomp directory:
make all and make clean

From top level from main Makefile:
make kselftest-install O=objdir ARCH=arm64 HOSTCC=gcc \
 CROSS_COMPILE=aarch64-linux-gnu- TARGETS=seccomp
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/seccomp/Makefile

 # SPDX-License-Identifier: GPL-2.0
-all:
-
-include ../lib.mk
-
-.PHONY: all clean
-
-BINARIES := seccomp_bpf seccomp_benchmark
 CFLAGS += -Wl,-no-as-needed -Wall
+LDFLAGS += -lpthread
 
-seccomp_bpf: seccomp_bpf.c ../kselftest_harness.h
-	$(CC) $(CFLAGS) $(LDFLAGS) $< -lpthread -o $@
-
-TEST_PROGS += $(BINARIES)
-EXTRA_CLEAN := $(BINARIES)
-
-all: $(BINARIES)
+TEST_GEN_PROGS := seccomp_bpf seccomp_benchmark
+include ../lib.mk