tools lib traceevent: Fixup jobserver setup

Getting rid of:

make[1]: Entering directory `/home/git/linux/tools/lib/traceevent'
make[2]: warning: jobserver unavailable: using -j1.  Add `+' to parent
make rule.
make[1]: Entering directory `/home/git/linux/tools/lib/lk

When running:

 make -j4 O=/tmp/build/perf -C tools/perf install-bin
Acked-by: Borislav Petkov <bp@suse.de>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/n/tip-pvr7uppe329gw9onchgdu0m6@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/traceevent/Makefile

@@ -60,7 +60,7 @@ ifeq ($(BUILD_SRC),)
 ifneq ($(BUILD_OUTPUT),)
 
 define build_output
-	$(if $(VERBOSE:1=),@)$(MAKE) -C $(BUILD_OUTPUT) 	\
+	$(if $(VERBOSE:1=),@)+$(MAKE) -C $(BUILD_OUTPUT) 	\
 	BUILD_SRC=$(CURDIR) -f $(CURDIR)/Makefile $1
 endef