Fix -Wmissing-variable-declarations warnings for float.c special case

Fix -Wmissing-variable-declarations warnings for float.c special case

This adds extern declarations for the global variables defined in
float.c but not meant for external use.  This is a workaround to be
able to add -Wmissing-variable-declarations to the global set of
warning options in the near future.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/flat/e0a62134-83da-4ba4-8cdb-ceb0111c95ce@eisentraut.org