Fixed compiler warning

```
 ./src/thread/pthread/SDL_syssem.c:140:12: warning: variable 'retval' is used uninitialized whenever 'while' loop exits because its condition is false [-Wsometimes-uninitialized]
    while (sem_trywait(&sem->sem) != 0) {
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~
./src/thread/pthread/SDL_syssem.c:149:12: note: uninitialized use occurs here
    return retval;
           ^~~~~~
./src/thread/pthread/SDL_syssem.c:140:12: note: remove the condition if it is always true
    while (sem_trywait(&sem->sem) != 0) {
```

This was a legitimate bug, thank you clang!

Fixes https://github.com/libsdl-org/SDL/issues/6830

src/thread/pthread/SDL_syssem.c

@@ -65,7 +65,7 @@ void SDL_DestroySemaphore(SDL_sem *sem)
 
 int SDL_SemWaitTimeoutNS(SDL_sem *sem, Sint64 timeoutNS)
 {
-    int retval;
+    int retval = 0;
 #ifdef HAVE_SEM_TIMEDWAIT
 #ifndef HAVE_CLOCK_GETTIME
     struct timeval now;