[fix] issue 7-undefined reference to clock_gettime

platform:
rela@yijie-ThinkPad-L412:~/work/dysonltd-gts$ uname -a
Linux yijie-ThinkPad-L412 3.2.0-55-generic #85-Ubuntu SMP Wed Oct 2 12:29:27 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux

compiler:
rela@yijie-ThinkPad-L412:~/work/dysonltd-gts$ gcc --version
gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3
Copyright (C) 2011 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.