btrfs-progs: fix build, manpage compression command

btrfs-progs: fix build, manpage compression command

man pages for btrfs-progs are compressed by gzip by default. In Makefile
the variable GZIP is use, this evaluates to 'gzip gzip' on my system.
>From man gzip:

> The environment variable GZIP can hold a set of default options for
> gzip. These options are interpreted first and can be overwritten by
> explicit command line parameters.

So using any other variable name fixes this.

Signed-off-by: Christian Hesse <mail@eworm.de>
Signed-off-by: David Sterba <dsterba@suse.cz>