cmake: Deduplicate _list_prefix macro

The macro is only used after an identical version (_gir_list_prefix)
is included with GObjectIntrospectionMacros.
11 jobs for master in 9 minutes and 16 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #5053823
build

00:04:27

passed #5053828
build_android

00:00:41

passed #5053824
build_clang_libcpp

00:02:36

passed #5053827
build_clazy

00:02:53

passed #5053826
build_mingw64_fedora

00:02:07

passed #5053825
build_ubuntu_16_04

00:01:55

passed #5053822
clang_format

00:01:03

 
  Document
passed #5053830
cpp_docs

00:04:11

passed #5053831
glib_docs

00:04:33

passed #5053829
qt5_docs

00:04:19

 
  Publish
passed #5053832
trigger_pages

00:00:15