Don't assume that 'char' is signed.

On some platforms, notably ARM and PowerPC, 'char' is unsigned by
default. This fixes an assertion failure at WAL replay on such platforms.

Reported by Noah Misch. Backpatch to 9.5, where this was broken.

src/include/access/spgist_private.h

@@ -465,7 +465,7 @@ typedef struct spgxlogAddNode
 	 * -1: parent not updated
 	 *----
 	 */
-	char		parentBlk;
+	int8		parentBlk;
 	OffsetNumber offnumParent;	/* offset within the parent page */
 
 	uint16		nodeI;