Fix MSVC warnings from new timezone code

Fix MSVC warnings from new timezone code

The new timezone code from commit aeb07c55fab introduced a few new
compiler warnings from MSVC:

    ../src/timezone/zic.c(353): warning C5287: operands are different enum types '<unnamed-enum-RF_NAME>' and '<unnamed-enum-LF_TARGET>'; use an explicit cast to silence this warning
    ../src/timezone/zic.c(353): warning C5287: operands are different enum types '<unnamed-enum-RF_NAME>' and '<unnamed-enum-LP_YEAR>'; use an explicit cast to silence this warning
    ../src/timezone/zic.c(1654): warning C4146: unary minus operator applied to unsigned type, result still unsigned

Silence these warnings with pragmas.

In order not to make future code merges more complicated, the pragmas
apply to the entire file, not only to the specific code sections where
they are needed.  This also prevents further warnings of the same
kinds creeping in.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/2294297.1780270682%40sss.pgh.pa.us