trace-cmd: Fix Makefile list of PLUGIN_OBJS to be more readable

Instead of placing all the plugin objs in the variable to build on a
single line, use the += to add them. It looks much better.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
diff --git a/Makefile b/Makefile
index 09829d4..0dcaf91 100644
--- a/Makefile
+++ b/Makefile
@@ -319,9 +319,15 @@
 			trace-output.o trace-recorder.o trace-restore.o trace-usage.o \
 			trace-blk-hack.o kbuffer-parse.o
 
-PLUGIN_OBJS = plugin_hrtimer.o plugin_kmem.o plugin_sched_switch.o \
-	plugin_mac80211.o plugin_jbd2.o plugin_function.o plugin_kvm.o \
-	plugin_blk.o
+PLUGIN_OBJS =
+PLUGIN_OBJS += plugin_hrtimer.o
+PLUGIN_OBJS += plugin_kmem.o
+PLUGIN_OBJS += plugin_sched_switch.o
+PLUGIN_OBJS += plugin_mac80211.o
+PLUGIN_OBJS += plugin_jbd2.o
+PLUGIN_OBJS += plugin_function.o
+PLUGIN_OBJS += plugin_kvm.o
+PLUGIN_OBJS += plugin_blk.o
 
 PLUGINS := $(PLUGIN_OBJS:.o=.so)