x11/libxkbcommon: pet portlint via USE_LDCONFIG

$ portlint -C
WARN: /usr/ports/x11/libxkbcommon/pkg-plist: [10]: installing shared libraries, please define USE_LDCONFIG as appropriate
WARN: /usr/ports/x11/libxkbcommon/pkg-plist: [13]: installing shared libraries, please define USE_LDCONFIG as appropriate
WARN: /usr/ports/x11/libxkbcommon/pkg-plist: [16]: installing shared libraries, please define USE_LDCONFIG as appropriate
0 fatal errors and 3 warnings found.

PR: 254583
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D29664