Bug#1075965: libmpich-dev: libucx-dev dependency must not be unconditional

[Adrian Bunk]

Package: libmpich-dev
Version: 4.2.0-7
Severity: serious
Tags: ftbfs
X-Debbugs-Cc: Alastair McKinstry <mckinstry at debian.org>

hpcc build-depends on:
- mpi-default-dev:armhf
mpi-default-dev depends on:
- libmpich-dev:armhf
libmpich-dev depends on missing:
- libucx-dev:armhf


Before it was dropped by accident, the libucx-dev followed
the build dependency:
  libucx-dev [amd64  ppc64el arm64]


BTW:
Unrelated (perhaps for experimental):
Should libucx-dev also be used on riscv64, where it is now available?