Remove unneeded declarations from math_private.h

The declarations of many functions in math_private.h are not required
since __MATHDECL and __MATHDECLX, in math.h, already provide the
declarations for these functions.  This patch removes the declarations
from math_private.h. It also adds the inclusion of math.h to the files
which depended on the declaration of functions in math_private.h.

Tested for powerpc64le and s390x.

	* sysdeps/generic/math_private.h: Remove declarations of
	many functions that are already declared in math.h.
	* sysdeps/ieee754/ldbl-128/e_logl.c: Include math.h to get the
	declaration for __frexpl.
	* sysdeps/ieee754/ldbl-128ibm/e_logl.c: Include math.h to get
	the declarations for __scalbnl and fabsl.