testsuite: Fix up forwprop-39.c testcase [PR109145]

As written in the PR, newlib headers aren't C11 compliant in that they
don't define CMPLXF macro, and glibc before 2.16 doesn't define that
either.  I think it is easier to use __builtin_complex directly, over
another patch which keeps including complex.h but defines CMPLXF if it
isn't defined, we want to test how forwprop behaves rather than what
complex.h defines or doesn't define.

2023-03-17  Jakub Jelinek  <jakub@redhat.com>

	PR testsuite/109145
	* gcc.dg/tree-ssa/forwprop-39.c: Remove -std=c11 from dg-options.
	Don't include complex.h.
	(foo): Use __builtin_complex rather than CMPLXF.