package/procps-ng: Fix install, procps-ng does not use /usr/bin anymore

package/procps-ng: Fix install, procps-ng does not use /usr/bin anymore

We do not need to move binaries around anymore since this commit:
https://gitlab.com/procps-ng/procps/commit/430b559ba2826f80dffa840622ae0fc744000b13

Removing PROCPS_NG_CONF_OPTS completely leads to binaries being
installed to /usr/bin, we still have to define a custom --exec-prefix
to get binaries in /bin:

libtool: install: /usr/bin/install -c free /home/bernd/buildroot/output/target/bin/free
libtool: install: /usr/bin/install -c pgrep /home/bernd/buildroot/output/target/bin/pgrep
libtool: install: /usr/bin/install -c pkill /home/bernd/buildroot/output/target/bin/pkill
libtool: install: /usr/bin/install -c pmap /home/bernd/buildroot/output/target/bin/pmap
libtool: install: /usr/bin/install -c pwdx /home/bernd/buildroot/output/target/bin/pwdx
libtool: install: /usr/bin/install -c tload /home/bernd/buildroot/output/target/bin/tload
libtool: install: /usr/bin/install -c uptime /home/bernd/buildroot/output/target/bin/uptime
libtool: install: /usr/bin/install -c vmstat /home/bernd/buildroot/output/target/bin/vmstat
libtool: install: /usr/bin/install -c w /home/bernd/buildroot/output/target/bin/w
libtool: install: /usr/bin/install -c pidof /home/bernd/buildroot/output/target/bin/pidof
libtool: install: /usr/bin/install -c kill /home/bernd/buildroot/output/target/bin/kill
libtool: install: /usr/bin/install -c slabtop /home/bernd/buildroot/output/target/bin/slabtop
libtool: install: /usr/bin/install -c watch /home/bernd/buildroot/output/target/bin/watch
libtool: install: /usr/bin/install -c top/top /home/bernd/buildroot/output/target/bin/top

Fixes:

  http://autobuild.buildroot.net/results/ab7/ab7ed83d60823c1b023c7b76fcbe9c20d537ed4c//

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[Thomas: pass --exec-prefix=/ instead of an empty prefix.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>