qemu-block: fix building from distribution tarball when glib2-devel is installed

Building RPMs from a 'make dist' tarball fails when qemu-block is
enabled. Enabling is done automatically when the glib2 development files
are available (enabled by ./configure).

Manual building with:

  $ ./autogen.sh && ./configure && make dist && rpmbuild -ta *.tar.gz

Building in mock works fine, glib2-devel is not installed by default so
the qemu-block xlator gets disabled. This change also adds glib2-devel
to the BuildRequires in the glusterfs.spec file, causing the qemu-block
xlator to be built by default, and included in the glusterfs RPM.

Change-Id: Ibb73628772586d9e07bbfde7a8ff2fc973489086
BUG: 986775
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/5896
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>