pgsql: Add missing float.h include to snprintf.c.

Add missing float.h include to snprintf.c.

On windows _isnan() (which isnan() is redirected to in port/win32.h)
is declared in float.h, not math.h.

Per buildfarm animal currawong.

Backpatch to all supported branches.

Branch
------
REL9_4_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/742734d2b54005f228655f0733e869b16ec6cb2b

Modified Files
--------------
src/port/snprintf.c |    3 +++
1 file changed, 3 insertions(+)