tracing: small trave_events sample Makefile cleanup

Use -I$(src) to add the current directory the include path.

[ Impact: cleanup ]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

samples/trace_events/Makefile

 # builds the trace events example kernel modules;
 # then to use one (as root):  insmod <module_name.ko>
 
-PWD := $(shell pwd)
-
-CFLAGS_trace-events-sample.o := -I$(PWD)/samples/trace_events/
+CFLAGS_trace-events-sample.o := -I$(src)
 
 obj-$(CONFIG_SAMPLE_TRACE_EVENTS) += trace-events-sample.o