Fix function prototypes

Silences these compiler warnings:

    /pgxn/neon_walredo/walredoproc.c:452:1: warning: ‘CreateFakeSharedMemoryAndSemaphores’ was used with no prototype before its definition [-Wmissing-prototypes]
      452 | CreateFakeSharedMemoryAndSemaphores()
          | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    /pgxn/neon/walproposer_pg.c:541:1: warning: no previous prototype for ‘GetWalpropShmemState’ [-Wmissing-prototypes]
      541 | GetWalpropShmemState()
          | ^~~~~~~~~~~~~~~~~~~~

Part of the cleanup issue #9217.

pgxn/neon/walproposer.h

@@ -725,7 +725,7 @@ extern void WalProposerBroadcast(WalProposer *wp, XLogRecPtr startpos, XLogRecPt
 extern void WalProposerPoll(WalProposer *wp);
 extern void WalProposerFree(WalProposer *wp);
 
-extern WalproposerShmemState *GetWalpropShmemState();
+extern WalproposerShmemState *GetWalpropShmemState(void);
 
 /*
  * WaitEventSet API doesn't allow to remove socket, so walproposer_pg uses it to

pgxn/neon/walproposer_pg.c

@@ -538,7 +538,7 @@ nwp_shmem_startup_hook(void)
 }
 
 WalproposerShmemState *
-GetWalpropShmemState()
+GetWalpropShmemState(void)
 {
 	Assert(walprop_shared != NULL);
 	return walprop_shared;

pgxn/neon_walredo/walredoproc.c

@@ -136,7 +136,7 @@ static bool redo_block_filter(XLogReaderState *record, uint8 block_id);
 static void GetPage(StringInfo input_message);
 static void Ping(StringInfo input_message);
 static ssize_t buffered_read(void *buf, size_t count);
-static void CreateFakeSharedMemoryAndSemaphores();
+static void CreateFakeSharedMemoryAndSemaphores(void);
 
 static BufferTag target_redo_tag;
 
@@ -449,7 +449,7 @@ WalRedoMain(int argc, char *argv[])
  * half-initialized postgres.
  */
 static void
-CreateFakeSharedMemoryAndSemaphores()
+CreateFakeSharedMemoryAndSemaphores(void)
 {
 	PGShmemHeader *shim = NULL;
 	PGShmemHeader *hdr;