drd/tests/swapcontext.c: Rename typedef struct thread_local to threadlocal

drd/tests/swapcontext.c: Rename typedef struct thread_local to threadlocal

Since C23 thread_local is a keyword (thread storage duration).

swapcontext.c:23:16: error: expected '{' before 'thread_local'
   23 | typedef struct thread_local {
      |                ^~~~~~~~~~~~
swapcontext.c:23:16: warning: 'thread_local' is not at beginning of declaration [-Wold-style-declaration]
swapcontext.c:23:16: error: 'thread_local' used with 'typedef'
swapcontext.c:26:3: warning: data definition has no type or storage class
   26 | } thread_local_t;
      |   ^~~~~~~~~~~~~~