common: Move gzip functions into a new gzip header

As part of the effort to remove things from common.h, create a new header
for the gzip functions. Move the function declarations to it and add
missing documentation.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>

fs/ubifs/ubifs.c

@@ -12,6 +12,7 @@
  */
 
 #include <common.h>
+#include <gzip.h>
 #include <memalign.h>
 #include "ubifs.h"
 #include <u-boot/zlib.h>