Fix "variable set but not used" warning with unnamed POSIX semaphores.

Late-model clang is able to notice, when building posix_sema.c with
USE_NAMED_POSIX_SEMAPHORES not defined, that the static variable
nextSemKey is set once and read nowhere.  Rearrange the #ifdef's
to not declare or set it unless USE_NAMED_POSIX_SEMAPHORES.  As a
small side benefit, we can skip a stat() call altogether if we're
not going to need it to initialize this variable, which I think
clarifies the code a fraction.

Per buildfarm member fritillary.

src/backend/port/posix_sema.c

@@ -65,7 +65,10 @@ static PGSemaphore sharedSemas; /* array of PGSemaphoreData in shared memory */
 #endif
 static int	numSems;			/* number of semas acquired so far */
 static int	maxSems;			/* allocated size of above arrays */
+
+#ifdef USE_NAMED_POSIX_SEMAPHORES
 static int	nextSemKey;			/* next name to try */
+#endif
 
 
 static void ReleaseSemaphores(int status, Datum arg);
@@ -197,6 +200,7 @@ PGSemaphoreShmemRequest(int maxSemas)
 void
 PGSemaphoreInit(int maxSemas)
 {
+#ifdef USE_NAMED_POSIX_SEMAPHORES
 	struct stat statbuf;
 
 	/*
@@ -210,6 +214,8 @@ PGSemaphoreInit(int maxSemas)
 				(errcode_for_file_access(),
 				 errmsg("could not stat data directory \"%s\": %m",
 						DataDir)));
+	nextSemKey = statbuf.st_ino;
+#endif
 
 #ifdef USE_NAMED_POSIX_SEMAPHORES
 	mySemPointers = (sem_t **) malloc(maxSemas * sizeof(sem_t *));
@@ -219,7 +225,6 @@ PGSemaphoreInit(int maxSemas)
 
 	numSems = 0;
 	maxSems = maxSemas;
-	nextSemKey = statbuf.st_ino;
 
 	on_shmem_exit(ReleaseSemaphores, 0);
 }