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/include/pgtar.h

@@ -0,0 +1,15 @@
+/*-------------------------------------------------------------------------
+ *
+ * pgtar.h
+ *	  Functions for manipulating tarfile datastructures (src/port/tar.c)
+ *
+ *
+ * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/pgtar.h
+ *
+ *-------------------------------------------------------------------------
+ */
+extern void tarCreateHeader(char *h, const char *filename, const char *linktarget, size_t size, mode_t mode, uid_t uid, gid_t gid, time_t mtime);
+extern int	tarChecksum(char *header);