Add fake gcc-4.5 package built with equivs.

Workaround for:

  linux-headers-3.0.0-1-something is needed to build virtualbox-guest-dkms
modules.
  linux-headers-3.0.0-1-something depends on gcc-4.5, because this is the default
  kernel compiler for Debian kernels since 3.0.0~rc2-1~experimental.1.
  gcc-4.5 is unavailable on Squeeze.

Hopefully a proper Linux 3.0.x backport will be uploaded soon, and we can get
rid of this crude hack.