[ob/6.0] Fix a new sparc build failure

[Andrew Cagney]

>> Can you please move the extern declaration to "sparc-tdep.h"?  Without 
>> it, there's yet another -Wmissing-prototypes to fix.
> 
> 
> Would you prefer creating a sparc-tdep.h for this, or just moving the
> prototype out of the #if?  There's no sparc-tdep.h yet and I don't want
> to do surgery on the sparc port to populate it properly.

Might was well just be done with it and create the "sparc-tdep.h" file. 
  No reason to populate it with anything other than something to fix the 
immediate problem though :-)

Andrew