repart: Fix compilation warning when tpm2 is disabled

repart: Fix compilation warning when tpm2 is disabled

The warning is about unused variable "flags":

```
../src/partition/repart.c: In function ‘partition_encrypt’:
../src/partition/repart.c:3690:19: warning: unused variable ‘flags’ [-Wunused-variable]
 3690 |         TPM2Flags flags = 0;
      |                   ^~~~~
```

Move the flags variable into the scope where it is actually used, which happens
to be inside the HAVE_TPM2 block.