cpplib.h (struct cpp_callbacks): Annotate error with ATTRIBUTE_FPTR_PRINTF(3,0) instead of ATTRIBUTE_PRINTF(3,0).

	* include/cpplib.h (struct cpp_callbacks): Annotate error with
	ATTRIBUTE_FPTR_PRINTF(3,0) instead of ATTRIBUTE_PRINTF(3,0).

Co-Authored-By: Ian Lance Taylor <ian@airs.com>

From-SVN: r106891