testsuite: Fix prototype in gcc.dg/pr114115.c

testsuite: Fix prototype in gcc.dg/pr114115.c

One test failing with a -std=gnu23 default that I wanted to
investigate further is gcc.dg/pr114115.c.  Building with -std=gnu23
produces a warning:

pr114115.c:18:8: warning: 'ifunc' resolver for 'foo_ifunc2' should return 'void * (*)(void)' [-Wattribute-alias=]

It turns out that this warning (from cgraphunit.cc) is disabled for
unprototyped functions.  Fix the return type for foo_ifunc2 so the
test builds without warnings both with and without -std=gnu23.

Tested for x86_64.

	* gcc.dg/pr114115.c (foo_ifunc2): Return void.