snippets: don't use G_GNUC_CONST for get_type() functions.

According to the GCC documentation [1], const is stricter than pure and
should not be used with anything that modifies global memory.

[1] https://gcc.gnu.org/onlinedocs/gcc-3.1/gcc/Function-Attributes.html

https://bugzilla.gnome.org/show_bug.cgi?id=738557