core: drop unnecessary __useless_struct_to_allow_trailing_semicolon__

ISO C does not allow empty statements outside of functions, and gcc
will warn the trailing semicolons when compiling with -pedantic:

  warning: ISO C does not allow extra ‘;’ outside of a function [-Wpedantic]

But our code cannot compile with -pedantic anyway, at least because

  warning: ISO C does not support ‘__PRETTY_FUNCTION__’ predefined identifier [-Wpedantic]

Without -pedatnic, clang and even old gcc (3.4) generate no warnings about
those semicolons, so let's just drop __useless_struct_to_allow_trailing_semicolon__.

src/basic/nss-util.h

@@ -116,8 +116,7 @@ enum nss_status _nss_##module##_gethostbyname_r(        \
                         NULL,                           \
                         NULL);                          \
        return ret;                                      \
-}                                                       \
-struct __useless_struct_to_allow_trailing_semicolon__
+}
 
 #define NSS_GETHOSTBYADDR_FALLBACKS(module)             \
 enum nss_status _nss_##module##_gethostbyaddr_r(        \
@@ -133,8 +132,7 @@ enum nss_status _nss_##module##_gethostbyaddr_r(        \
                         buffer, buflen,                 \
                         errnop, h_errnop,               \
                         NULL);                          \
-}                                                       \
-struct __useless_struct_to_allow_trailing_semicolon__
+}
 
 #define NSS_GETPW_PROTOTYPES(module)                    \
 enum nss_status _nss_##module##_getpwnam_r(             \