Use fsverity_formatted_digest from <linux/fsverity.h>

struct fsverity_formatted_digest (previously called
fsverity_signed_digest) is now in <linux/fsverity.h>, so there is no
longer any need to have a local definition of it.

Test: build
Change-Id: Ie3623a56fe6415d686a51ddfde8a1ebab83b8364
diff --git a/ondevice-signing/VerityUtils.cpp b/ondevice-signing/VerityUtils.cpp
index 318345a..56dcd5e 100644
--- a/ondevice-signing/VerityUtils.cpp
+++ b/ondevice-signing/VerityUtils.cpp
@@ -50,13 +50,6 @@
 #define le16_to_cpu(v) (__builtin_bswap16((__force uint16_t)(v)))
 #endif
 
-struct fsverity_signed_digest {
-    char magic[8]; /* must be "FSVerity" */
-    __le16 digest_algorithm;
-    __le16 digest_size;
-    __u8 digest[];
-};
-
 static std::string toHex(std::span<uint8_t> data) {
     std::stringstream ss;
     for (auto it = data.begin(); it != data.end(); ++it) {
@@ -121,7 +114,7 @@
 
 static Result<std::vector<uint8_t>> signDigest(const SigningKey& key,
                                                const std::vector<uint8_t>& digest) {
-    auto d = makeUniqueWithTrailingData<fsverity_signed_digest>(digest.size());
+    auto d = makeUniqueWithTrailingData<fsverity_formatted_digest>(digest.size());
 
     memcpy(d->magic, "FSVerity", 8);
     d->digest_algorithm = cpu_to_le16(FS_VERITY_HASH_ALG_SHA256);