fix multiple definition

Build tests failed caused by multiple definition of test_offset
```
../library/libpolarssl.a(ctr_drbg.c.o): In function `_GLOBAL__sub_D_00099_0_ctr_drbg_init_entropy_len':
/polarssl/library/ctr_drbg.c:456: multiple definition of `test_offset'
CMakeFiles/test_suite_ctr_drbg.dir/test_suite_ctr_drbg.c.o:/polarssl/build/tests/test_suite_ctr_drbg.c:11: first defined here
```