lib: just copy the TPMU_HA union instead of copying its member

The TPMU_HA is just a union data type, so instead of attempting to copy
.na field, copy the union since both refer to the same memory location.

This also allows the tpm2-tools to be built against the tpm2-tss 1.x
since the TPMU_HA union doesn't have a .na field in that version. So it
prevents the following build error:

lib/tpm2_alg_util.c: In function ‘tpm2_extract_plain_signature’:
lib/tpm2_alg_util.c:348:33: error: ‘TPMU_HA {aka union <anonymous>}’ has no member named ‘na’
         memcpy(buffer, &hmac_sig->na, *size);
                                 ^~
make: *** [Makefile:1871: lib/tpm2_alg_util.o] Error 1

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>

lib/tpm2_alg_util.c

@@ -351,7 +351,7 @@ UINT8* tpm2_extract_plain_signature(UINT16 *size, TPMT_SIGNATURE *signature) {
         if (!buffer) {
             goto nomem;
         }
-        memcpy(buffer, &hmac_sig->na, *size);
+        memcpy(buffer, &hmac_sig, *size);
         break;
     }
     case TPM_ALG_ECDSA: {