Export SDL_LinuxSetThreadPriorityAndPolicy() function (#4877)

It's marked as being a public symbol internally, however, it was missing from the header files and not visible in the shared library.  This adds it to the necessary headers and to the DynAPI list to expose it for use by applications.

Co-authored-by: Frank Praznik <frank.praznik@oh.rr.com>

src/thread/pthread/SDL_systhread.c

@@ -183,17 +183,6 @@ SDL_ThreadID(void)
     return ((SDL_threadID) pthread_self());
 }
 
-#if __LINUX__
-/**
-   \brief Sets the SDL priority (not nice level) for a thread, using setpriority() if appropriate, and RealtimeKit if available.
-   Differs from SDL_LinuxSetThreadPriority in also taking the desired scheduler policy,
-   such as SCHED_OTHER or SCHED_RR.
-
-   \return 0 on success, or -1 on error.
- */
-extern DECLSPEC int SDLCALL SDL_LinuxSetThreadPriorityAndPolicy(Sint64 threadID, int sdlPriority, int schedPolicy);
-#endif
-
 int
 SDL_SYS_SetThreadPriority(SDL_ThreadPriority priority)
 {