Move tar function headers to pgtar.h

This makes it possible to include them only where they are used, so
we can avoid the conflict of the uid_t and gid_t datatypes that happened
in plperl (since plperl doesn't need the tar functions)

src/port/tar.c

@@ -1,4 +1,5 @@
 #include "c.h"
+#include "pgtar.h"
 #include <sys/stat.h>
 
 /*