Fix compiler warnings on macOS (#9319)

## Problem

On macOS:
```
/Users/runner/work/neon/neon//pgxn/neon/file_cache.c:623:19: error: variable 'has_remaining_pages' is used uninitialized whenever 'for' loop exits because its condition is false [-Werror,-Wsometimes-uninitialized]
```

## Summary of changes
- Initialise `has_remaining_pages` with `false`

pgxn/neon/file_cache.c

@@ -618,7 +618,7 @@ lfc_evict(NRelFileInfo rinfo, ForkNumber forkNum, BlockNumber blkno)
 	 */
 	if (entry->bitmap[chunk_offs >> 5] == 0)
 	{
-		bool		has_remaining_pages;
+		bool		has_remaining_pages = false;
 
 		for (int i = 0; i < CHUNK_BITMAP_SIZE; i++)
 		{