gh-38859: build/pkgs/matroid_database: mark as math

    
From passagemath/passagemath#183:

'Currently it appears in the "Other dependencies" section in
https://doc.sagemath.org/html/en/reference/spkg/; it should go in the
"Math" section.'
    
URL: #38859
Reported by: gmou3
Reviewer(s): Kwankyu Lee