nldbl-compat.c: Include math.h before nldbl-compat.h.

Jeff Law noticed that native PowerPC builds were broken by my having
made math_ldbl_opt.h not include math.h.  nldbl-compat.c formerly got
math.h via libioP.h and math_ldbl_opt.h, *without* __NO_LONG_DOUBLE_MATH;
after my change it got it via nldbl-compat.h *with* __NO_LONG_DOUBLE_MATH,
but __NO_LONG_DOUBLE_MATH mode is forbidden on hosts that define
__HAVE_DISTINCT_FLOAT128, so the build breaks.  This is the quick fix.

	* sysdeps/ieee754/ldbl-opt/nldbl-compat.c: Include math.h
	before nldbl-compat.h.