meson.build: Fix generating pkg-config files

Hi,

From the commit message:

On Windows and Cygwin builds, we need to put the '.exe' for our entries into the pkg-config files for 'g-ir-compiler' and 'g-ir-generate', otherwise Meson will not be able to identify these programs correctly.

This was caused by the recent changes in g-i that we use the Meson's built-in methods to generate the pkg-config files, but the .exe extension was forgotten, in the commit that was merged by MR !207 (merged).

With blessings, thank you!