package/pinentry: fix build with Qt4

Help configure find the correct moc, as the comment in m4/qt.m4 states:

    #configure tried to run $ac_cv_path_moc and the test didn't
    #succeed. If configure shouldn't have tried this one, set
    #the environment variable MOC to the right one before running
    #configure.

Fixes:
    http://autobuild.buildroot.org/results/06c/06c5c2762ec22271fec20a6d15548011e40875d1
    http://autobuild.buildroot.org/results/0d7/0d78a2ff8c952cd104794f5049b07216978fdf11
    http://autobuild.buildroot.org/results/af8/af8f7f588dfa64d82c98a4b42ec78fd17dde137c

Also fixes using the host moc:
    http://autobuild.buildroot.org/results/836/836962f23dcad77b145fcdfeffb40e38e64d6971



Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>