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/dynapi/SDL_dynapi_overrides.h

@@ -824,3 +824,4 @@
 #define SDL_vasprintf SDL_vasprintf_REAL
 #define SDL_GetWindowICCProfile SDL_GetWindowICCProfile_REAL
 #define SDL_GetTicks64 SDL_GetTicks64_REAL
+#define SDL_LinuxSetThreadPriorityAndPolicy SDL_LinuxSetThreadPriorityAndPolicy_REAL

src/dynapi/SDL_dynapi_procs.h

@@ -891,3 +891,6 @@ SDL_DYNAPI_PROC(int,SDL_asprintf,(char **a, SDL_PRINTF_FORMAT_STRING const char
 SDL_DYNAPI_PROC(int,SDL_vasprintf,(char **a, const char *b, va_list c),(a,b,c),return)
 SDL_DYNAPI_PROC(void*,SDL_GetWindowICCProfile,(SDL_Window *a, size_t *b),(a,b),return)
 SDL_DYNAPI_PROC(Uint64,SDL_GetTicks64,(void),(),return)
+#ifdef __LINUX__
+SDL_DYNAPI_PROC(int,SDL_LinuxSetThreadPriorityAndPolicy,(Sint64 a, int b, int c),(a,b,c),return)
+#endif