Disable the warning about deprecation inside qalgorithms.h

Since some of the algorithms use other ones, we should not warn about
those. The warnings are supposed to happen only in user code.

Warnings obtained with GCC 5. The Clang change is just to be on the safe
side.

Change-Id: If295899f6ff6534de7b19741d33efc0b5c4c912c
Reviewed-by: Robin Burchell <robin.burchell@viroteck.net>