elfcompress: Don't compress if section already compressed unless forced

elfcompress: Don't compress if section already compressed unless forced

Before commit a5b07cdf9 "support ZSTD compression algorithm"
elfcompress would not try to compress a section if it already
had the requested compression type (or was already uncompressed)
unless the --force flag was given. An else if construct was changed
to an if in the commit causing elfcompress to warn (in verbose mode)
but then still try to (re)compress the section.

Add an explicit check so if nothing needs (un)compressing, the file
isn't changed.

The diff looks large, but git diff -b -w is just:

+     if (force || type != schtype)
+       {
          if (shdr->sh_type != SHT_NOBITS
              && (shdr->sh_flags & SHF_ALLOC) == 0)
            {
@@ -554,6 +556,7 @@ process_file (const char *fname)
              printf ("[%zd] %s ignoring %s section\n", ndx, sname,
                      (shdr->sh_type == SHT_NOBITS ? "no bits" : "allocated"));
        }
+   }

Signed-off-by: Mark Wielaard <mark@klomp.org>