debian: Ensure the gmp build-dep comes before the relicensing

In version 4.2.2 released in September 2007, GMP switched from the LGPL-2.1+ license to LGPL-3+ OR GPL-2, which makes it no longer compliant with the Apertis licensing policies, since proprietary applications cannot link to it without having to comply with the problematic GPL-3 terms.

To avoid that, in Apertis we ship an old LGPL-2.1 version of GMP in the gmp4 package and the current gmp package no longer provides the libgmp3-dev binary.

To further reduce the chance of building against the wrong version, let's require here a version that pre-dates the licensing change.