meson: include whole src/lib for eo file generation in ifaces

meson: include whole src/lib for eo file generation in ifaces

In autotools, src/lib is used as an include path for finding
eo files, which is the same behavior you would get as when using
a system directory. In meson, this was changed to include only
certain directories.

This presents a problem (05682eed7d572874928424af82fba819790c4228)
as it is not possible to reference symbols from other paths within
documentation.

Since this is necessary for the time being, work around the problem
by doing a whole src/lib include for now, which fixes the parsing.
Longer term solution is to be discussed and implemented.