build: Add missing rpmbuild/BUILD dir for CentOS 5.x

The rpmbuild/BUILD directory isn't created automatically
by the build system on CentOS 5.x. This 1-line patch
adds its creation to the build process.

Backport of http://review.gluster.org/#/c/7175/

BUG: 1071504
Change-Id: I2cd6cab3a84aa8e163ee75b69f7609c74a163e78
Signed-off-by: Justin Clift <justin@gluster.org>
Reviewed-on: http://review.gluster.org/7298
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>