iconv/loop.c: Fix -Wundef warning with !_STRING_ARCH_unaligned

iconv/loop.c: Fix -Wundef warning with !_STRING_ARCH_unaligned

If code is required to handle the unaligned case then loop.c includes
itself and relies on the #undefs at the end of the file to avoid
outputting two copies of LOOPFCT and gconv_btowc. However
MAX_NEEDED_INPUT is tested with #if so this causes a warning.
Reorder the code so that the function definitions are in an #else
block to make the behaviour clearer and fix the warning.

Verified that code is unchanged on x86_64 and arm.

ChangeLog:

2014-07-17  Will Newton  <will.newton@linaro.org>

	* iconv/loop.c: Move definition of LOOPFCT and gconv_btowc
	into an #else block.