src: Remove $(builddir) usage

src: Remove $(builddir) usage

Commit 4e8032272f1704f7 used $(builddir) in the header search
path to fix a build issue; however, $(builddir) is not defined
by old autoconf versions such as the one available in CentOS 5,
resulting in the following error:

  cc1: error: /util: No such file or directory
  make[3]: *** [libvirt_driver_la-fdstream.lo] Error 1

Since $(builddir) is defined to always be '.', just use that
value directly instead.