libperf: Add libperf dependency for tests targets

Add libperf dependency for tests targets.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lore.kernel.org/lkml/20190913132355.21634-36-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/lib/Makefile

@@ -138,7 +138,7 @@ clean: $(LIBAPI)-clean
                 *.o *~ *.a *.so *.so.$(VERSION) *.so.$(LIBPERF_VERSION) .*.d .*.cmd LIBPERF-CFLAGS $(LIBPERF_PC)
 	$(Q)$(MAKE) -C tests clean
 
-tests:
+tests: libs
 	$(Q)$(MAKE) -C tests
 	$(Q)$(MAKE) -C tests run