sdjournal: drop conditional static specifier

`_SD_ARRAY_STATIC` is a custom / conditional macro for static C99
array size, which got introduced in recent libsystemd and thus cannot
build on older versions.
Due to how it is currently used in our Go wrapper, it can be safely
omitted from here.

Ref: systemd/systemd@4199f68

sdjournal/journal.go

@@ -310,7 +310,7 @@ package sdjournal
 // }
 //
 // char *
-// my_sd_id128_to_string(void *f, sd_id128_t boot_id, char s[_SD_ARRAY_STATIC SD_ID128_STRING_MAX])
+// my_sd_id128_to_string(void *f, sd_id128_t boot_id, char s[SD_ID128_STRING_MAX])
 // {
 //   char *(*sd_id128_to_string)(sd_id128_t, char *);
 //