graphics/mapnik: fix build on armv7, prepare for boost 1.89

Add a missing cast from uint64_t to size_t.
Remove LIB_DEPENDS on libboost_system, which no longer exists
on boost 1.89 (it has been a shim before).

Approved by: portmgr (build fix blanket)
MFH: 2025Q3

(cherry picked from commit dd0ccaf9dd47b0738b9ca84ced8265d8e837cbd4)