Previously, the base math headers included GMP headers in all cases.
This was problematic because we don't want all modules that use the
math headers to depend on GMP, and the unnecessary includes could
theoretically have detrimental effects to compile times.
Now `BLI_math_mpq.hh` depends on `BLI_math_base.hh`, so if a file
needs to use exact arithmatic, it can just include the former.
Differential Revision: https://developer.blender.org/D15079
3060b98842