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_0_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/00331a0f23f25944d495260796c29778649a5041

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