p11-kit: fix __progname build failure

Even though we've got UCLIBC_HAS___PROGNAME=y in our internal toolchains
the build seems to fail under some circumstances.
Since program_invocation_short_name is equally as useful just disable
__progname usage. Fixes:
http://autobuild.buildroot.net/results/cc8/cc861f5a034a0922a90fb6f22835a1fe8205d497/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>