build: Fix requires in pc files

When xres or libstartup-notification are not available, they were
still being listed in Requires.private field of the pkg-config file.
This is a regression in Meson build files compared to Autotools.
2 jobs for wip/jtojnar/meson-fixes in 9 minutes and 40 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #390852
build:ubuntu:autogen

00:09:40

passed #390853
build:ubuntu:meson

00:02:45