Include <threads.h> if possible to get thread_local definition

IN C23, thread_local is a reserved keyword and we shall therefore
do nothing to redefine it. glibc has it defined for older standard
version with the right conditions.

v2 by Yu Watanabe:
Move the definition to missing_threads.h like the way we define e.g.
missing syscalls or missing definitions, and include it by the users.

Co-authored-by: Yu Watanabe <watanabe.yu+github@gmail.com>

src/basic/process-util.c

@@ -36,6 +36,7 @@
 #include "memory-util.h"
 #include "missing_sched.h"
 #include "missing_syscall.h"
+#include "missing_threads.h"
 #include "mountpoint-util.h"
 #include "namespace-util.h"
 #include "nulstr-util.h"