Build: libpe_status: Expose internal library functions for unit testing

If we are building libpe_status_test, undefine G_GNUC_INTERNAL so
functions using this macro are globally visible. Otherwise, the
functions will be hidden and nothing linking to libpe_status_test will
be able to see them. This makes unit testing them impossible.

Adapted from a0e2f4b

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>