FindThreads: Fix pthreads detection with -Wmissing-prototypes

If the project builds with `-Wmissing-prototypes` to catch unnecessarily
exported symbols, `CheckForPthreads.c` will fail to build.  Fix this by
marking `start_routine` as static.

Fixes: #25886

Modules/CheckForPthreads.c

@@ -1,6 +1,6 @@
 #include <pthread.h>
 
-void* start_routine(void* args)
+static void* start_routine(void* args)
 {
   return args;
 }