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 7 minutes and 51 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #1277958
build

00:03:36

passed #1277963
build_android

00:00:55

passed #1277959
build_clang_libcpp

00:07:50

passed #1277962
build_clazy

00:02:35

passed #1277961
build_mingw64_fedora

00:04:39

passed #1277960
build_ubuntu_16_04

00:01:46