psa: Unused key_bits is OK

When MD or CMAC are disabled, let the compiler know that it is OK that
`key_bits` is set but not used by casting `key_bits` to `(void)`.
diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index 291dcdb..d100eb1 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -1869,6 +1869,7 @@
     else
 #endif /* MBEDTLS_MD_C */
     {
+        (void) key_bits;
         status = PSA_ERROR_NOT_SUPPORTED;
     }