tools: Fix pthread flag for Makefile of trace-agent used by virtio-trace

pthread flag should not be -lpthread but -pthread using gcc. The -lpthread
links the external multithread library. On the other hand, the -pthread manages
both the gcc's preprocessor and linker to be able to compile with pthread.
Signed-off-by: Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@hitachi.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tools/virtio/virtio-trace/Makefile

 CC = gcc
-CFLAGS = -O2 -Wall
-LFLAG = -lpthread
+CFLAGS = -O2 -Wall -pthread
 
 all: trace-agent
 
 .c.o:
-	$(CC) $(CFLAGS) $(LFLAG) -c $^ -o $@
+	$(CC) $(CFLAGS) -c $^ -o $@
 
 trace-agent: trace-agent.o trace-agent-ctl.o trace-agent-rw.o
-	$(CC) $(CFLAGS) $(LFLAG) -o $@ $^
+	$(CC) $(CFLAGS) -o $@ $^
 
 clean:
 	rm -f *.o trace-agent