dumper: Fix missing symbols in LTO case or with overridden symbol visibility settings

In case a user had a combination of -fvisibility=hidden, -Wl,--as-needed, -flto, -O2 in the CFLAGS the linker would sometimes detect that all the referenced gtype functions weren't actually used and throw them out with their providing libraries.

Instead of hoping that the user's CFLAGS don't mess without our symbol visibility just use G_MODULE_EXPORT on the two symbols which reference all other gtype and gquark symbols.

This fixes errors such as:

Invalid GType function: 'gtk_accel_group_get_type' Failed to find symbol 'gtk_accel_group_get_type'

during the g-ir-scanner execution.

Fixes #280 (closed)