package/ipmiutil: do not build doc

Fixes:
    http://autobuild.buildroot.org/results/2ec/2ecfb1143ba89ffa5cdc8096bb175b2c396c4670/
    http://autobuild.buildroot.org/results/c49/c497fc446140694084922d51fe6be308ce5c1c1a/
    http://autobuild.buildroot.org/results/434/434b156b5c9b5c7b65ffe6174cf4e029e7e3ffd8/



Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>