add gobject to Requires: line

vips C and C++ programs can link directly against things in gobject
without appearing to, for example:

	a = VIPS_OBJECT( b );

the macro will expand to include a g_type_check_instance_is_a(). So we
have to have gobject-2.0 in the link line of things like:

	g++ -g -Wall hello8.cc `pkg-config vips-cpp --cflags --libs`

see also

	https://github.com/jcupitt/libvips/issues/196