x11/libXpm: update to 3.5.15
Changelogs:
3.5.14: https://gitlab.freedesktop.org/xorg/lib/libxpm/-/merge_requests/5
3.5.15: https://gitlab.freedesktop.org/xorg/lib/libxpm/-/merge_requests/10
PR: 270485
Reviewed by: danfe, jbeich, manu
MFH: 2023Q1
Security: 38f213b6-8f3d-4067-91ef-bf14de7ba518