gnupg2: fix dependency on libpthsem

Commit beb49d88 ('gnupg2: new
package') was incorrect in its dependency on libpthsem: it was
selecting BR2_PACKAGE_PTHSEM while it should select
BR2_PACKAGE_LIBPTHSEM. This commit fixes that.

Fixes:

  http://autobuild.buildroot.org/results/6e0/6e02b42f01986a56c22c44658fa1e488c0a08e7c/



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