Fix a compile error on Windows.

Write the array literal of all zeros as {0} rather than {}.

Change-Id: If15330d96d019be671d3bcbbdea60c2b3ecc2128
Reviewed-on: https://boringssl-review.googlesource.com/2740
Reviewed-by: Adam Langley <agl@google.com>

crypto/hkdf/hkdf_test.c

@@ -101,8 +101,10 @@ static const hkdf_test_vector_t kTests[] = {
       0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b,
     }, 22,
     {
+       0,
     }, 0,
     {
+       0,
     }, 0,
     42, {
       0x8d, 0xa4, 0xe7, 0x75, 0xa5, 0x63, 0xc1, 0x8f, 0x71, 0x5f, 0x80, 0x2a,
@@ -176,8 +178,10 @@ static const hkdf_test_vector_t kTests[] = {
       0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b,
     }, 22,
     {
+       0,
     }, 0,
     {
+       0,
     }, 0,
     42, {
       0x0a, 0xc1, 0xaf, 0x70, 0x02, 0xb3, 0xd7, 0x61, 0xd1, 0xe5, 0x52, 0x98,
@@ -193,8 +197,10 @@ static const hkdf_test_vector_t kTests[] = {
       0x0c, 0x0c, 0x0c, 0x0c, 0x0c, 0x0c, 0x0c, 0x0c, 0x0c, 0x0c,
     }, 22,
     {
+       0,
     }, 0,
     {
+       0,
     }, 0,
     42, {
       0x2c, 0x91, 0x11, 0x72, 0x04, 0xd7, 0x45, 0xf3, 0x50, 0x0d, 0x63, 0x6a,