make install: LIBEXECDIR should be an absolute path

[marcov]

LIBEXECDIR should be made an absolute path, to comply with the GNU Make interpretation and to be consistent with make install commands used for the other kata components.

Current behavior for proxy:

make install DESTDIR=/tmp/foobar PREFIX=/usr LIBEXECDIR=/usr/libexec 
install -D kata-proxy /tmp/foobar///usr//usr/libexec/kata-containers/kata-proxy

For comparison, runtime:

$ make install V=1 DESTDIR=/tmp/foobar PREFIX=/usr LIBEXECDIR=/usr/libexec
[omitted lines]
install -D kata-runtime /tmp/foobar/usr/bin/kata-runtime;
install -D kata-netmon /tmp/foobar/usr/libexec/kata-containers/kata-netmon;