multipath-tools: avoid using GZIP Makefile variable

`gzip` supports (deprecated) `GZIP` environment variable. If it's
already present Makefile would override it and pass it through causing
thre breakage:

    $ dev>GZIP=-n make
    gzip -9 -c mpath_persistent_reserve_in.3 > mpath_persistent_reserve_in.3.gz
    gzip: -c: option not valid in GZIP environment variable
    Try `gzip --help' for more information.

Fix build by renaming GZIP variable to GZIP_PROG to avoid collision.

mwilck: most of this is obsoleted by a220dd1 ("build: don't compress man pages")

CC: Martin Wilck <mwilck@suse.com>
CC: Benjamin Marzinski <bmarzins@redhat.com>
Signed-off-by: Sergei Trofimovich <slyich@gmail.com>
Signed-off-by: Martin Wilck <mwilck@suse.com>
Reviewed-by: Martin Wilck <mwilck@suse.com>

Makefile.inc

@@ -96,7 +96,7 @@ nvmedir		= $(TOPDIR)/libmultipath/nvme
 includedir	= $(prefix)/usr/include
 pkgconfdir	= $(usrlibdir)/pkgconfig
 
-GZIP		= gzip -9 -c
+GZIP_PROG	= gzip -9 -c
 RM		= rm -f
 LN		= ln -sf
 INSTALL_PROGRAM	= install