Use escaped version of typeof

When compiling code that includes gobject.h using GCC with the ISO
standard, the `typeof` keyword is disabled, as it's a GCC extension.

The GCC documentation recommends:

> If you are writing a header file that must work when included in
> ISO C programs, write __typeof__ instead of typeof.

Which is precisely what we're going to do.

Signed-off-by: Emmanuele Bassi <ebassi@gnome.org>
Reviewed-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=790697