BUILD: 51d: fix build issue with recent compilers

With gcc-11.2 and binutils-2.37 I'm getting link errors due to multiply
defined symbols when enabling USE_51DEGREES_V4. This is caused by two
variables being present in hash.h instead of hash.c, hence they're
defined twice.

This patch just moves them to hash.c and turns their declaration to
extern.

No backport is needed since this was introduced in 2.8-dev.
2 files changed