sd-journal: silence bogus gcc warning

In function 'sd_id128_equal',
    inlined from 'journal_file_verify' at ../src/libsystemd/sd-journal/journal-verify.c:1047:29:
../src/systemd/sd-id128.h:119:43: error: 'entry_boot_id.qwords[0]' may be used uninitialized [-Werror=maybe-uninitialized]
  119 |         return a.qwords[0] == b.qwords[0] && a.qwords[1] == b.qwords[1];
      |                ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/libsystemd/sd-journal/journal-verify.c: In function 'journal_file_verify':
../src/libsystemd/sd-journal/journal-verify.c:823:20: note: 'entry_boot_id.qwords[0]' was declared here
  823 |         sd_id128_t entry_boot_id;
      |                    ^~~~~~~~~~~~~
cc1: all warnings being treated as errors

entry_boot_id is only used when entry_monotonic_set has been set, and that's
only done in one place where entry_boot_id is also initalized.

src/journal/journalctl.c

@@ -2164,7 +2164,7 @@ int main(int argc, char *argv[]) {
         bool previous_boot_id_valid = false, first_line = true, ellipsized = false, need_seek = false;
         bool use_cursor = false, after_cursor = false;
         _cleanup_(sd_journal_closep) sd_journal *j = NULL;
-        sd_id128_t previous_boot_id;
+        sd_id128_t previous_boot_id = {};  /* Unnecessary initialization to appease gcc */
         int n_shown = 0, r, poll_fd = -1;
 
         setlocale(LC_ALL, "");