tests/qtest/tdisp-test: fix warning

/home/user/.work/qemu/tests/qtest/tdisp-test.c: In function ‘tdisp_testdev_send_message’:
/home/user/.work/qemu/tests/qtest/tdisp-test.c:214:30: error: ‘index’ may be used uninitialized [-Werror=maybe-uninitialized]
  214 |     for (size_t index; index < message_size / sizeof(uint32_t); ++index) {
      |                        ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/user/.work/qemu/tests/qtest/tdisp-test.c:214:17: note: ‘index’ was declared here
  214 |     for (size_t index; index < message_size / sizeof(uint32_t); ++index) {
      |                 ^~~~~
cc1: all warnings being treated as errors

Signed-off-by: Pierrick Bouvier <[email protected]>

tests/qtest/tdisp-test.c

@@ -211,7 +211,7 @@ static libspdm_return_t tdisp_testdev_send_message(
         return LIBSPDM_STATUS_SEND_FAIL;
     }
 
-    for (size_t index; index < message_size / sizeof(uint32_t); ++index) {
+    for (size_t index = 0; index < message_size / sizeof(uint32_t); ++index) {
         qpcie_config_writel(&dev->dev, PCI_EXP_DOE + PCI_EXP_DOE_WR_DATA_MBOX,
                             data[index]);
     }