Add missing const qualifier

This is present in the declaration for ReadDataFromArchive, so we'd better
have it in the definition too in order to avoid compilers from complaining
about the mismatch of function signatures.

src/bin/pg_dump/compress_io.c

@@ -124,7 +124,8 @@ AllocateCompressor(const pg_compress_specification compression_spec,
  * out with ahwrite().
  */
 void
-ReadDataFromArchive(ArchiveHandle *AH, pg_compress_specification compression_spec,
+ReadDataFromArchive(ArchiveHandle *AH,
+					const pg_compress_specification compression_spec,
 					ReadFunc readF)
 {
 	if (compression_spec.algorithm == PG_COMPRESSION_NONE)