usb: tools: fix a regression issue that gcc can't link to pthread

Reproduce:
ray@hr-bak:~/usb$ make -C tools/usb/
make: Entering directory `/home/ray/usb/tools/usb'
gcc -Wall -Wextra -g -lpthread -I../include -o testusb testusb.c
/tmp/cc0EMxfy.o: In function `main':
/home/ray/usb/tools/usb/testusb.c:508: undefined reference to `pthread_create'
/home/ray/usb/tools/usb/testusb.c:531: undefined reference to `pthread_join'
collect2: error: ld returned 1 exit status
make: *** [testusb] Error 1
make: Leaving directory `/home/ray/usb/tools/usb'

Comments:
In the latest version (4.7.3) of gcc compiler, it requres that
libraries must follow the object or source files like below:

"gcc hello.c -lpthread" instead of "gcc -lpthread hello.c"

And it isn't encountered at gcc version 4.7.2.
So this patch fix to move the pthread option after testusb.c.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>