[Freetel-codec2] Minor change to codec2-dev

[Steve]

In freedv_api_internal.h

change:

#ifndef __FREEDV__

to

#ifndef __FREEDV_INTERNAL__

http://sourceforge.net/p/freetel/code/HEAD/tree/codec2-dev/src/freedv_api_internal.h#l38

Reason:

__FREEDV__ is already used in freedv_api.h

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