mpdecimal: bump to 2.4.1 and add hash file

Bumping mpdecimal to 2.4.1 is necessary to make the Python 3 'decimal'
module to build properly, since Python 3.4.2. Thanks to Samuel Martin
for the investigation!

Fixes:

  http://autobuild.buildroot.org/results/c9b/c9b6dd19f16a56a16ef441d5b303badab9720bd7/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>