sd-bus: bus_message_type_from_string is not pure

sd-bus: bus_message_type_from_string is not pure

GCC document [1] says:

    The pure attribute prohibits a function from modifying the state
    of the program that is observable by means other than inspecting
    the function’s return value.

And there is an example:

    `int hash (char *) __attribute__ ((pure));`
    ... Even though hash takes a non-const pointer argument it must
    not modify the array it points to, ...

But we are modifying the object pointed to by the pointer u, which is
clearly a violation of the semantic of pure.

With -ftrivial-auto-var-init (enabled by -Dmode=release), on some
targets (GCC 12.2 on AArch64 and GCC 13.1 on x86_64) performs an
optimization: as the variable "u" in bus_match_parse has been
zero-initialized (by the -ftrivial-auto-var-init option) and never
modified (because a "pure" bus_message_type_from_string is not allowed
to modify it), "u" will be always 0.

Then 0 is used to initialize .value_u8 field of struct
bus_match_component.  This then causes a infinite event loop, so
"systemctl restart" never stops, and pam_systemd timeouts communicating
with logind, etc.

So we should remove the "pure" attribute here.

Fixes #26395.

[1]:https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-pure-function-attribute