Re-add SPICleanup for ABI compatibility in stable branch

This fixes an ABI break introduced by
604651880c.

Author: Markus Wanner <markus.wanner@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/defd749a-8410-841d-1126-21398686d63d@enterprisedb.com

src/backend/executor/spi.c

@@ -422,6 +422,16 @@ SPI_rollback_and_chain(void)
 	_SPI_rollback(true);
 }
 
+/*
+ * SPICleanup is a no-op, kept for backwards compatibility. We rely on
+ * AtEOXact_SPI to cleanup. Extensions should not (need to) fiddle with the
+ * internal SPI state directly.
+ */
+void
+SPICleanup(void)
+{
+}
+
 /*
  * Clean up SPI state at transaction commit or abort.
  */

src/include/executor/spi.h

@@ -205,6 +205,7 @@ extern void SPI_commit_and_chain(void);
 extern void SPI_rollback(void);
 extern void SPI_rollback_and_chain(void);
 
+extern void SPICleanup(void);
 extern void AtEOXact_SPI(bool isCommit);
 extern void AtEOSubXact_SPI(bool isCommit, SubTransactionId mySubid);
 extern bool SPI_inside_nonatomic_context(void);