selftests/bpf: Honor $(O) when figuring out paths

When building the kselftests out-of-tree, e.g. ...

| make ARCH=riscv CROSS_COMPILE=riscv64-linux-gnu- \
| O=/tmp/kselftest headers
| make ARCH=riscv CROSS_COMPILE=riscv64-linux-gnu- \
| O=/tmp/kselftest HOSTCC=gcc FORMAT= \
| SKIP_TARGETS="arm64 ia64 powerpc sparc64 x86 sgx" \
| -C tools/testing/selftests gen_tar

... the kselftest build would not pick up the correct GENDIR path, and
therefore not including autoconf.h.

Correct that by taking $(O) into consideration when figuring out the
GENDIR path.

Signed-off-by: Björn Töpel <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/Makefile

···
 TOOLSINCDIR := $(TOOLSDIR)/include
 BPFTOOLDIR := $(TOOLSDIR)/bpf/bpftool
 APIDIR := $(TOOLSINCDIR)/uapi
+ifneq ($(O),)
+GENDIR := $(O)/include/generated
+else
 GENDIR := $(abspath ../../../../include/generated)
+endif
 GENHDR := $(GENDIR)/autoconf.h
 HOSTPKG_CONFIG := pkg-config