Fix mistake in XLOG_SEG_SIZE test.

The intent of the test is to check whether XLOG_SEG_SIZE is in a
particular range, but actually in one case it compares XLOG_BLCKSZ
by mistake.  Repair.

Commit 88e982302684246e8af785e78a467ac37c76dee9 introduced this
faulty test.

Kuntal Ghosh, reviewed by Michael Paquier.

src/backend/utils/misc/guc.c

@@ -707,7 +707,7 @@ typedef struct
 #if XLOG_BLCKSZ < 1024 || XLOG_BLCKSZ > (1024*1024)
 #error XLOG_BLCKSZ must be between 1KB and 1MB
 #endif
-#if XLOG_SEG_SIZE < (1024*1024) || XLOG_BLCKSZ > (1024*1024*1024)
+#if XLOG_SEG_SIZE < (1024*1024) || XLOG_SEG_SIZE > (1024*1024*1024)
 #error XLOG_SEG_SIZE must be between 1MB and 1GB
 #endif