Fix implicit __isinf declarations in exp.

My recent exp patch introduced warnings about implicit __isinf
declarations in exp because e_exp.c didn't include <math.h>.  This
patch fixes this.  Because <math.h> can't be included after
<math_private.h> (because of macro definitions of __nan*), it was
necessary to put an include in sysdeps/x86_64/fpu/multiarch/e_exp.c as
well.

Tested x86_64.

	* sysdeps/ieee754/dbl-64/e_exp.c: Include <math.h>.
	* sysdeps/x86_64/fpu/multiarch/e_exp.c
	[HAVE_FMA4_SUPPORT || HAVE_AVX_SUPPORT]: Likewise.