pdftoppm: Compile fix for the unsupported UTILS_USE_PTHREADS define

7 jobs for UTILS_USE_PTHREADS in 5 minutes and 24 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #7644557
build

00:03:39

passed #7644562
build_android

00:00:50

passed #7644558
build_clang_libcpp

00:03:48

passed #7644561
build_clazy

00:05:22

passed #7644560
build_mingw64_fedora

00:02:23

passed #7644559
build_ubuntu_16_04

00:02:01

passed #7644556
clang_format

00:01:07