package/gensio: add libglib2 optional dependency

libglib2 is an optional dependency which is enabled by default since
version 2.3.0 and
https://github.com/cminyard/gensio/commit/866924d69dabe8b8a981e19bd745d5c84e529881

Handle it properly to avoid the following build failure since bump to
version 2.5.2 in commit 5bb747f0:

sparc64-buildroot-linux-gnu-gcc: ERROR: unsafe header/library path used in cross-compilation: '-I/usr/include/glib-2.0'

Fixes:
 - http://autobuild.buildroot.org/results/3602c5cc2aff668211fd8e6eeab778b3fe287246



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>