fix longstanding error in pthread_attr_t definition

[cfriedt]

Describe the bug
It looks like there was a copy-paste error made several years ago when defining pthread_attr_t.

c8aa657

typedef struct pthread_attr_t {
...
} pthread_attr_t;

To Reproduce
Discovered during CI build of #25527

Expected behavior
Expected build to succeed

Impact
Blocks #25527

Screenshots or console output
https://app.shippable.com/github/zephyrproject-rtos/zephyr/runs/73710/1/console

pthread_attr_t defined as wrong kind of tag
...
conflicting types for pthread_attr_t

Environment (please complete the following information):
CI (see shippable link)

Additional context

[cfriedt]

Even though it's a legitimate typo, it looks like it might be specific to the nrf52_bsim.
This kind of gives it away:

-include /home/buildslave/src/github.com/zephyrproject-rtos/zephyr/arch/posix/include/posix_cheats.h

I should adjust my #ifdefs accordingly in #25527