Fix aarch64/104201: branch-protection-attr.c fails after quoting difference

After the quoting changes in r12-6521-g03a1a86b5ee40d4e240, branch-protection-attr.c
fails due to expecting a different quoting type for "leaf".
This patch changes the quoting from "" to '' as that is what is used now.

Committed as obvious after a test of the testcase.

gcc/testsuite/ChangeLog:

	PR target/104201
	* gcc.target/aarch64/branch-protection-attr.c: Fix quoting for
	the expected error message on line 5 of leaf.