Install gpartedbin into @libexecdir@

Quoting the relevant commit message:

Executables which are not intended for execution by users, but by other
programs, should be installed into /usr/libexec [1][2].  gpartedbin
falls into this category.  Update it's installation accordingly.

Standard Autotools details: gpartedbin will be installed into
EPREFIX/libexec by default.  To install gpartedbin into a different
directory set libexecdir when configuring the build system.  Like this
from git:
    ./autogen.sh --libexecdir=DIR
or like this from tar release:
    ./configure --libexecdir=DIR

[1] Filesystem Hierarchy Standard, version 3.0,
    4.7. /usr/libexec : Binaries run by other programs (optional)
    https://refspecs.linuxfoundation.org/FHS_3.0/fhs/ch04s07.html
    "/usr/libexec includes internal binaries that are not intended to be
    executed directly by users or shell scripts.
    "

[2] GNU Coding Standards, June 12, 2020,
    7.2.5 Variables for Installation Directories
    https://www.gnu.org/prep/standards/html_node/Directory-Variables.html
    "libexecdir
    The directory for installing executable programs to be run by other
    programs rather than by users.  This directory should normally be
    /usr/local/libexec, but write it as $(exec_prefix)/libexec.  (If you
    are using Autoconf, write it as '@libexecdir@'.)
    "

Closes #85 (closed) - Please install gpartedbin under /usr/libexec instead of /usr/sbin