package/libfribidi: add optional dependency on libglib2

Fix the following message when libglib2 is enabled but not already
built:
Package glib-2.0 was not found in the pkg-config search path.
Perhaps you should add the directory containing `glib-2.0.pc'
No package 'glib-2.0' found

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>