build: Fix inexact calculation of required materials
sector_can_build() computes mat[i] * (effic / 100.0). The division is
inexact. The result gets randomly rounded, so errors are vanishingly
unlikely to screw up material consumption.
However, we require the amount rounded up to be present since commit
1227d2c. Errors *can* screw that up. Fix by avoiding inexact
computation for that part.
We should probably review rounding of inexact values in general.
Signed-off-by: Markus Armbruster <armbru@pond.sub.org>