Ensure to include float.h to use DBL_* constants

[tadd]

Hello 👋 Long time no see.

Thanks for your simple and easy to use library. We use this all the time to organize our research data.

Here is a simple one-line PR to include <float.h> to fix a warning: "DBL_MANT_DIG" is not defined, evaluates to 0 [-Wundef].
This is because the file statics.c uses DBL_MANT_DIG but no proper header <float.h> included, which is in the standard C.