irc/irssi-xmpp: fix build on powerpc64 elfv2

LLd10 10 fails with:
ld: error: relocation R_PPC64_TOC16_LO cannot be used against symbol read_settings; recompile with -fPIC

defined in xmpp-settings.o
referenced by xmpp-settings.c

xmpp-settings.o:(xmpp_settings_deinit)