lftp: bump version to 4.7.2

lftp: bump version to 4.7.2

Help lftp to find readline and zlib so it will not fail this way:

checking for Readline... no
configure: error: cannot find readline library, install readline-devel
package

checking if zlib is wanted... yes
checking for inflateEnd in -lz... no
checking zlib.h usability... no
checking zlib.h presence... no
checking for zlib.h... no
configure: error: cannot find -lz library, install zlib-devel package

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
[Thomas: group zlib/readline options into one definition of
LFTP_CONF_OPTS.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>