[ARM] nwfpe: fix 'floatx80_is_nan' sparse warning

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>

arch/arm/nwfpe/softfloat.h

@@ -226,6 +226,8 @@ char floatx80_le_quiet( floatx80, floatx80 );
 char floatx80_lt_quiet( floatx80, floatx80 );
 char floatx80_is_signaling_nan( floatx80 );
 
+extern flag floatx80_is_nan(floatx80);
+
 #endif
 
 static inline flag extractFloat32Sign(float32 a)