Switch Confidence from float to double to align with global.c

[DavisVaughan]

This should fix this LTO warning

extern.h:72:14: warning: type of 'Confidence' does not match original declaration [-Wlto-type-mismatch]
   72 | extern float Confidence, SampleFrac, *Vote, *BVoteBlock, **MCost, **NCost,
      |              ^
global.c:138:8: note: type 'double' should match type 'float'
  138 | double Confidence; /* set by classify() */
      |        ^
global.c:138:8: note: 'Confidence' was previously declared here
global.c:138:8: note: code may be misoptimized unless '-fno-strict-aliasing' is used

It looks like it was supposed to be a double based on its usage in classify.c