backends: Fix warning from Sparse

Sparse report:

backends/tpm.c:39:5: warning: returning void-valued expression
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

backends/tpm.c

@@ -36,7 +36,7 @@ void tpm_backend_destroy(TPMBackend *s)
 {
     TPMBackendClass *k = TPM_BACKEND_GET_CLASS(s);
 
-    return k->ops->destroy(s);
+    k->ops->destroy(s);
 }
 
 int tpm_backend_init(TPMBackend *s, TPMState *state,