meson: use / instead of join_paths() for readability

4 jobs for fix-msys2-compiler-warning in 6 minutes and 20 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #3713784
debian sid

00:01:14

passed #3713787
gstreamer-windows 1809 windows docker
msys2

00:03:57

passed #3713785
gstreamer-windows 1809 windows docker
vs2017 amd64

00:01:06

passed #3713786
gstreamer-windows 1809 windows docker
vs2017 x86

00:01:02