package/brltty: use gcc instead of ld to link shared objects

Fixes:
  http://autobuild.buildroot.net/results/2419741c43dea0f91859c19167c8d65cb54e1470
  http://autobuild.buildroot.net/results/4b83e53c718520661dc99cde2121f69ce3fd08d7



Signed-off-by: Mario Lang <mlang@blind.guru>
[Thomas: add reference to upstream commit, as suggested by Yann.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>