The symbol 'floatx80_is_nan' prototype was defined
locally in fpa11_cprt.c when it was built outside the
file in softfloat-specialisze.

Move this into softfloat.h to fix the following sparse
warning:

softfloat-specialize:276:6: warning: symbol 'floatx80_is_nan' was not declared. Should it be static?
Signed-off-by: Ben Dooks <ben-linux@fluff.org>