src/erasure-code/jerasure/CMakeLists.txt:: add -Wno-unused-but-set-variable

src/erasure-code/jerasure/CMakeLists.txt:: add -Wno-unused-but-set-variable

```
[460/1945] Building C object src/erasure-code/jerasure/CMakeFiles/jerasure_objs.dir/jerasure/src/jerasure.c.o
ceph/src/erasure-code/jerasure/jerasure/src/jerasure.c:722:7: warning: variable 'ddf' set but not used [-Wunused-but-set-variable]
  722 |   int ddf, cdf;
      |       ^
ceph/src/erasure-code/jerasure/jerasure/src/jerasure.c:722:12: warning: variable 'cdf' set but not used [-Wunused-but-set-variable]
  722 |   int ddf, cdf;
      |            ^
ceph/src/erasure-code/jerasure/jerasure/src/jerasure.c:777:7: warning: variable 'ddf' set but not used [-Wunused-but-set-variable]
  777 |   int ddf, cdf;
      |       ^
ceph/src/erasure-code/jerasure/jerasure/src/jerasure.c:777:12: warning: variable 'cdf' set but not used [-Wunused-but-set-variable]
  777 |   int ddf, cdf;
      |            ^
4 warnings generated.
```

Signed-off-by: Matan Breizman <mbreizma@redhat.com>