Fix another incorrect data type choice from commit dc2123400.

add_file_to_manifest declared its mtime argument as pg_time_t,
apparently on the principle that copy-and-paste from the backend
is fine.  However, the callers are passing struct stat's st_mtime
field which is plain time_t, and add_file_to_manifest itself is
passing the value to gmtime(3) which expects plain time_t,
so the whole thing would not work at all on any platform where
those types are different.  Fortunately we can just switch this
variable to time_t.

Per warnings from assorted buildfarm members.

src/bin/pg_combinebackup/write_manifest.c

@@ -72,7 +72,7 @@ create_manifest_writer(char *directory)
  */
 void
 add_file_to_manifest(manifest_writer *mwriter, const char *manifest_path,
-					 size_t size, pg_time_t mtime,
+					 size_t size, time_t mtime,
 					 pg_checksum_type checksum_type,
 					 int checksum_length,
 					 uint8 *checksum_payload)

src/bin/pg_combinebackup/write_manifest.h

@@ -13,7 +13,6 @@
 #define WRITE_MANIFEST_H
 
 #include "common/checksum_helper.h"
-#include "pgtime.h"
 
 struct manifest_wal_range;
 
@@ -23,7 +22,7 @@ typedef struct manifest_writer manifest_writer;
 extern manifest_writer *create_manifest_writer(char *directory);
 extern void add_file_to_manifest(manifest_writer *mwriter,
 								 const char *manifest_path,
-								 size_t size, pg_time_t mtime,
+								 size_t size, time_t mtime,
 								 pg_checksum_type checksum_type,
 								 int checksum_length,
 								 uint8 *checksum_payload);