Static libraries don't include dependency information, so when using
pkg-config to find out how to link with a static library, the .pc file
for that library has to specify what other libraries to link.
Libs.private is like Libs, except it's only used when static linking.
This fixes linking against a static libpsl using pkg-config.

The Meson-generated libpsl.pc just includes all of its dependent
libraries in Libs, so doesn't require any additional fix.