gnutls: Fix build for Linuxbrew
gnutls: Fix build for Linuxbrew

Disable postinstall.

Closes #620.

Signed-off-by: Shaun Jackman <b580dab3251a9622aba3803114310c23fdb42900@gmail.com>

DoomHammer/linuxbrew,ebouaziz/linuxbrew,ebouaziz/linuxbrew,Angeldude/linuxbrew,SuperNEMO-DBD/cadfaelbrew,bukzor/linuxbrew,bukzor/linuxbrew,drbenmorgan/linuxbrew,Linuxbrew/linuxbrew,DoomHammer/linuxbrew,Angeldude/linuxbrew,ebouaziz/linuxbrew,gnubila-france/linuxbrew,gnubila-france/linuxbrew,saketkc/linuxbrew,Linuxbrew/linuxbrew,Linuxbrew/linuxbrew,SuperNEMO-DBD/cadfaelbrew,gnubila-france/linuxbrew,bukzor/linuxbrew,sjackman/linuxbrew,Homebrew/linuxbrew,drbenmorgan/linuxbrew,Angeldude/linuxbrew,reelsense/linuxbrew,saketkc/linuxbrew,ebouaziz/linuxbrew,SuperNEMO-DBD/cadfaelbrew,Homebrew/linuxbrew,Homebrew/linuxbrew,reelsense/linuxbrew,gnubila-france/linuxbrew,DoomHammer/linuxbrew,saketkc/linuxbrew,saketkc/linuxbrew,sjackman/linuxbrew,SuperNEMO-DBD/cadfaelbrew,reelsense/linuxbrew,drbenmorgan/linuxbrew,drbenmorgan/linuxbrew,Linuxbrew/linuxbrew,sjackman/linuxbrew,Angeldude/linuxbrew,DoomHammer/linuxbrew,bukzor/linuxbrew,reelsense/linuxbrew,Homebrew/linuxbrew,sjackman/linuxbrew