[PATCH] benchtests: Create objdir in the bench-%.c generation rule

The $(objpfx)bench-%.c rule writes its output into $(objpfx) without
ensuring that directory exists.  Serial builds happened to satisfy
that ordering, with parallel builds the generation recipe can
run before the directory is created, failing with:

  cannot create .../benchtests/bench-xxx.c-tmp: Directory nonexistent

Add the standard $(make-target-directory).
---
 benchtests/Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/benchtests/Makefile b/benchtests/Makefile
index f407e492cb7..16cc951d195 100644
--- a/benchtests/Makefile
+++ b/benchtests/Makefile
@@ -622,6 +622,7 @@ $(bench-link-targets): %: %.o $(objpfx)json-lib.o \
 $(bench-link-targets): LDFLAGS += $(link-bench-bind-now)
 
 $(objpfx)bench-%.c: %-inputs $(bench-deps)
+	$(make-target-directory)
 	{ if [ -n "$($*-INCLUDE)" ]; then \
 	  cat $($*-INCLUDE); \
 	fi; \
-- 
2.53.0