Include math.h in nextafter-2.c test.

Since the test is compiled with -fno-builtin, include math.h to allow for
implementations (like the PowerPC) that have multiple versions of long double
that are selectable by switch.  Math.h could possibly switch what function
nextafterl points to.

gcc/testsuite/
2020-11-17  Michael Meissner  <meissner@linux.ibm.com>

	* gcc.dg/nextafter-2.c: Include math.h.