tools build: Correct libsubcmd fixdep dependencies

All built targets need fixdep to be built first, before handling object
dependencies [1]. We're missing one such dependency before the libsubcmd
target.

This resolves .cmd file generation issues such that the following
sequence produces many fewer results:

$ git clean -xfd tools/
$ make tools/objtool
$ grep "cannot find fixdep" $(find tools/objtool -name '*.cmd')

In particular, only a buggy tools/objtool/libsubcmd/.fixdep.o.cmd
remains, due to circular dependencies of fixdep on itself.

Such incomplete .cmd files don't usually cause a direct problem, since
they're designed to fail "open", but they can cause some subtle problems
that would otherwise be handled by proper fixdep'd dependency files. [2]

[1] This problem is better described in commit abb26210a395 ("perf
tools: Force fixdep compilation at the start of the build"). I don't
apply its solution here, because additional recursive make can be a bit
of overkill.

[2] Example failure case:

cp -arl linux-src linux-src2
cd linux-src2
make O=/path/to/out
cd ../linux-src
rm -rf ../linux-src2
make O=/path/to/out

Previously, we'd see errors like:

make[6]: *** No rule to make target
'/path/to/linux-src2/tools/include/linux/compiler.h', needed by
'/path/to/out/tools/bpf/resolve_btfids/libsubcmd/exec-cmd.o'. Stop.

Now, the properly-fixdep'd .cmd files will ignore a missing
/path/to/linux-src2/...

Signed-off-by: Brian Norris <[e-mail]>
Acked-by: Jiri Olsa <[e-mail]>
Cc: Ian Rogers <[e-mail]>
Cc: Josh Poimboeuf <[e-mail]>
Cc: Masahiro Yamada <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Richter <[e-mail]>
Link: https://lore.kernel.org/all/ZGVi9HbI43R5trN8@bhelgaas/
Link: https://lore.kernel.org/all/[e-mail]/
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/lib/subcmd/Makefile

···
 
 all: fixdep $(LIBFILE)
 
-$(SUBCMD_IN): FORCE
+$(SUBCMD_IN): fixdep FORCE
 	@$(MAKE) $(build)=libsubcmd
 
 $(LIBFILE): $(SUBCMD_IN)