games/devilutionX: fix empty credits window

Fix empty "Show Credits" window by changing TTF_FONT_PATH
to TTF_FONT_DIR/TTF_FONT_NAME, introduced in v1.1.0 [1].

[1] https://github.com/diasurgical/devilutionX/commit/791aa60cab453899dfaa7456cde1e8c23fcbee79

PR: 252601
Submitted by: Nuno Teixeira <ed.arrakis@gmail.com> (maintainer)
Approved by: arrowd, tcberner (mentors, implicit)