postinstall: fix freebsd x86 wrapper path

Spotted in the FreeBSD Port (http://svnweb.freebsd.org/ports/head/security/i2p/files/patch-installer__resources__postinstall.sh?view=markup)
merge of '1202f5d3645add930a96dfa77cd2b2ebfb56ca95'
     and '5c131297d763f8953029fea257681d55f2103aaa'