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 5 minutes and 18 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #7463020
build

00:05:18

passed #7463025
build_android

00:00:58

passed #7463021
build_clang_libcpp

00:05:11

passed #7463024
build_clazy

00:04:56

passed #7463023
build_mingw64_fedora

00:03:12

passed #7463022
build_ubuntu_16_04

00:03:20

passed #7463019
clang_format

00:01:14