configure: fix duplicate of -lrt flag

'-lrt' flag duplication would cause 'undefined reference to clock_gettime' error
during compilation time.

Before fix:
... -o qemu-bridge-helper qemu-bridge-helper.o    -lrt -pthread -lgthread-2.0 -lrt -lglib-2.0
After fix:
... -o qemu-bridge-helper qemu-bridge-helper.o    -pthread -lgthread-2.0 -lrt -lglib-2.0

Reference:
http://hi.baidu.com/sanitywolf/item/7a8b69c1e76dd220a0b50ab1