textproc/rubygem-nokogiri: fix build on armv7

Add USES=pkgconfig to fix the build on arm.

rubygem-pkg-config crashes on armv7 for some reason unknown to me.
Luckily, pkgconfig is tried first and if present, succeeds in detecting
libxml2.

Approved by: portmgr (build fix blanket)
MFH: 2025Q4