build: Fix generating .pc file when using built-in loader(s)

Hi,

From the commit message:

The current Meson build files define built-in loader(s) as a dependency to libgdkpixbuf, which has an undesired side effect of making them required by GDK-Pixbuf's pkg-config file, which renders build systems that consume this pkg-config file output bad build files.

This fixes this by:

• Make libgdkpixbuf depend on the objects that are built for those loader(s), when the loader(s) are built into libgdkpixbuf.
• Include the external dependencies of these built-in loaders, if applicable, such as libpng etc., as dependencies of libgdkpixbuf.

With blessings, thank you!