textproc/scdoc: fix prefix in pkg-config

scdoc's homegrown build doesn't adjust pkg-config's prefix which
breaks .path() in meson. Found via x11-wm/sway upstream:

Dependency scdoc found: YES 1.9.0
Program /usr/bin/scdoc found: NO

meson.build:100:1: ERROR: Program(s) ['/usr/bin/scdoc'] not found or not executable

Reported by: @emersion