tools/virtio: compile with -pthread

When using pthreads, one has to compile and link with -lpthread,
otherwise e.g. glibc is not guaranteed to be reentrant.

This replaces -lpthread.
Reported-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

tools/virtio/Makefile

@@ -5,7 +5,8 @@ virtio_test: virtio_ring.o virtio_test.o
 vringh_test: vringh_test.o vringh.o virtio_ring.o
 
 CFLAGS += -g -O2 -Werror -Wno-maybe-uninitialized -Wall -I. -I../include/ -I ../../usr/include/ -Wno-pointer-sign -fno-strict-overflow -fno-strict-aliasing -fno-common -MMD -U_FORTIFY_SOURCE -include ../../include/linux/kconfig.h
-LDFLAGS += -lpthread
+CFLAGS += -pthread
+LDFLAGS += -pthread
 vpath %.c ../../drivers/virtio ../../drivers/vhost
 mod:
 	${MAKE} -C `pwd`/../.. M=`pwd`/vhost_test V=${V}