Use the correct type for function passed to pthread_create.

[gordonmessmer]

Description

This patch modifies the thread function in two test files, so that it matches the expected prototype for an argument to pthread_create. This fixes a build failure with gcc 15:

heap-non-main.c: In function ‘main’:
heap-non-main.c:27:40: error: passing argument 3 of ‘pthread_create’ from incompatible pointer type [-Wincompatible-pointer-types]
   27 |         pthread_create(&thread1, NULL, thread, NULL);
      |                                        ^~~~~~
      |                                        |
      |                                        void * (*)(void)
In file included from heap-non-main.c:8:
/usr/include/pthread.h:204:36: note: expected ‘void * (*)(void *)’ but argument is of type ‘void * (*)(void)’
  204 |                            void *(*__start_routine) (void *),
      |                            ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
heap-non-main.c:11:7: note: ‘thread’ declared here
   11 | void *thread()
      |       ^~~~~~

Checklist

• My code follows the code style of this project.
• My change includes a change to the documentation, if required.
• If my change adds new code, adequate tests have been added.
• I have read and agree to the CONTRIBUTING document.

[github-actions]

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