testsuite: Fix up pr109524.C for -std=c++23 [PR109524]

This testcase was reduced such that it isn't valid C++23, so with my
usual testing with GXX_TESTSUITE_STDS=98,11,14,17,20,2b it fails:
FAIL: g++.dg/pr109524.C  -std=gnu++2b (test for excess errors)
.../gcc/testsuite/g++.dg/pr109524.C: In function 'nn hh(nn)':
.../gcc/testsuite/g++.dg/pr109524.C:35:12: error: cannot bind non-const lvalue reference of type 'nn&' to an rvalue of type 'nn'
.../gcc/testsuite/g++.dg/pr109524.C:17:6: note:   initializing argument 1 of 'nn::nn(nn&)'
The following patch fixes that and I've verified it doesn't change
anything on what the test was testing, it still ICEs in r13-7198 and
passes in r13-7203, now in all language modes (except for 98 where
it is intentionally UNSUPPORTED).

2023-04-19  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/109524
	* g++.dg/pr109524.C (nn::nn): Change argument type from nn & to
	const nn &.

(cherry picked from commit 794ffdb0)