selftests/bpf: Add docs target as all dependency

Currently docs target is make dependency for TEST_GEN_FILES,
which makes tests to be rebuilt every time you run make.

Adding docs as all target dependency, so when running make
on top of built selftests it will show just:

$ make
make[1]: Nothing to be done for 'docs'.

After cleaning docs, only docs is rebuilt:

$ make docs-clean
CLEAN eBPF_helpers-manpage
CLEAN eBPF_syscall-manpage
$ make
GEN ...selftests/bpf/bpf-helpers.rst
GEN ...selftests/bpf/bpf-helpers.7
GEN ...selftests/bpf/bpf-syscall.rst
GEN ...selftests/bpf/bpf-syscall.2
$ make
make[1]: Nothing to be done for 'docs'.

Fixes: a01d935b2e09 ("tools/bpf: Remove bpf-helpers from bpftool docs")
Signed-off-by: Jiri Olsa <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/Makefile

···
 		    cp $(SCRATCH_DIR)/runqslower $@
 
 $(TEST_GEN_PROGS) $(TEST_GEN_PROGS_EXTENDED): $(OUTPUT)/test_stub.o $(BPFOBJ)
-$(TEST_GEN_FILES): docs
 
 $(OUTPUT)/test_dev_cgroup: cgroup_helpers.c
 $(OUTPUT)/test_skb_cgroup_id_user: cgroup_helpers.c
···
 		    EXTRA_CFLAGS='-g -Og'				       \
 		    OUTPUT=$(HOST_BUILD_DIR)/bpftool/			       \
 		    prefix= DESTDIR=$(HOST_SCRATCH_DIR)/ install
+
+all: docs
 
 docs:
 	$(Q)RST2MAN_OPTS="--exit-status=1" $(MAKE) $(submake_extras)	\