pgsql: Put back <float.h> in a few files that need it for _isnan().

Put back <float.h> in a few files that need it for _isnan().

Further fallout from commit c29aff959: there are some files that need
<float.h>, and were getting it from datatype/timestamp.h, but it was not
apparent in my (tgl's) testing because the requirement for <float.h>
exists only on certain Windows toolchains.

Report and patch by David Rowley.

Discussion: https://postgr.es/m/CAKJS1f-BHceaFzZScFapDV48gUVM2CAOBfhkgffdqXzFb+kwew@mail.gmail.com

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/86dbbf20d8496ede77566873d1e22cd11c1e544c

Modified Files
--------------
src/backend/access/gist/gistproc.c | 1 +
src/backend/access/gist/gistutil.c | 1 +
src/backend/utils/adt/orderedsetaggs.c | 1 +
src/backend/utils/adt/selfuncs.c | 1 +
4 files changed, 4 insertions(+)