poppler_annot_free_text_get_callout_line: Fix wrong static cast

static_cast always "suceeds" so it makes no sense using it to check if
the callout is multiline or not
7 jobs for glib_multiline in 4 minutes and 52 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #7538718
build

00:04:31

passed #7538723
build_android

00:02:50

passed #7538719
build_clang_libcpp

00:04:51

passed #7538722
build_clazy

00:03:39

passed #7538721
build_mingw64_fedora

00:02:53

passed #7538720
build_ubuntu_16_04

00:02:19

passed #7538717
clang_format

00:01:14