glib: rename signing_time to local_signing_time

Change name of the "signing_time" member of PopplerSignatureInfo
to "local_signing_time" which describes the value better.

Issue #464
6 jobs for glib-add-validation-of-signatures in 12 minutes and 21 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #860852
build

00:03:06

passed #860857
build_android

00:01:42

passed #860853
build_clang_libcpp

00:12:20

passed #860856
build_clazy

00:09:11

passed #860855
build_mingw64_fedora

00:01:53

passed #860854
build_ubuntu_16_04

00:02:32