opam: Fix for Linuxbrew
opam: Fix for Linuxbrew

Closes Linuxbrew/linuxbrew#977.

Signed-off-by: Bob W. Hogg <c772a964fd55352a3510e5d535dd9ccc9ac30168@linux.com>

SuperNEMO-DBD/cadfaelbrew,gnubila-france/linuxbrew,sjackman/linuxbrew,reelsense/linuxbrew,sjackman/linuxbrew,Linuxbrew/linuxbrew,reelsense/linuxbrew,sjackman/linuxbrew,SuperNEMO-DBD/cadfaelbrew,Angeldude/linuxbrew,saketkc/linuxbrew,Angeldude/linuxbrew,SuperNEMO-DBD/cadfaelbrew,gnubila-france/linuxbrew,saketkc/linuxbrew,Homebrew/linuxbrew,SuperNEMO-DBD/cadfaelbrew,Angeldude/linuxbrew,Angeldude/linuxbrew,Linuxbrew/linuxbrew,reelsense/linuxbrew,Homebrew/linuxbrew,saketkc/linuxbrew,gnubila-france/linuxbrew,Homebrew/linuxbrew,reelsense/linuxbrew,Linuxbrew/linuxbrew,Linuxbrew/linuxbrew,sjackman/linuxbrew,Homebrew/linuxbrew,saketkc/linuxbrew,gnubila-france/linuxbrew