php-gnupg: get tarball from sources.buildroot.net

pecl.php.net seems to have a database issue and returns a html page instead
of the tarball.

Fixes http://autobuild.buildroot.net/results/3b6/3b6c53ecbd8d25307ed77b29bffa62f5395f1981/



Signed-off-by: Peter Korsgaard <peter@korsgaard.com>