perf tools: Compare JOBS to 0 after grep

If JOBS is not by user perf tries to autodetect the number by grepping
the number of CPUs from /proc/cpuinfo. 'grep -c' will always return an
integer so after this command JOBS should be compared to 0, not "".
Signed-off-by: David Ahern <david.ahern@oracle.com>
Link: http://lkml.kernel.org/r/1424303971-91904-1-git-send-email-david.ahern@oracle.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Makefile

@@ -25,7 +25,7 @@ unexport MAKEFLAGS
 #
 ifeq ($(JOBS),)
   JOBS := $(shell grep -c ^processor /proc/cpuinfo 2>/dev/null)
-  ifeq ($(JOBS),)
+  ifeq ($(JOBS),0)
     JOBS := 1
   endif
 endif