Purge libglib2 dev files from target

* Don't install libglib2 development binaries and to target unless
  BR2_HAVE_DEVFILES is set

Signed-off-by: Malte Starostik <m-starostik@versanet.de>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>