x11/libei: add USE_LDCONFIG per portlint (nop)

/usr/local/lib is part of the default ldconfig_paths in rc.conf(5)

WARN: x11/libei/pkg-plist: [6]: installing shared libraries, please define USE_LDCONFIG as appropriate
WARN: x11/libei/pkg-plist: [9]: installing shared libraries, please define USE_LDCONFIG as appropriate
WARN: x11/libei/pkg-plist: [12]: installing shared libraries, please define USE_LDCONFIG as appropriate