Code does not compile if USE_FMS2_IO is undefined

[Hallberg-NOAA]

The dev/gfdl branch of the iceberg no longer compiles when USE_FMS2_IO is undefined, because the routine read_ice_sheet_basins() was only added to icebergs_fms2io.F90, and not to icebergs_fmsio.F90, but it is used regardless of which version of the FMS modules are being used.

This problem was introduced with #70, which was merged into the dev/gfdl branch of the icebergs code on August 21, 2024. The main branch of the icebergs code is not yet afflicted by this bug.

[alex-huth]

@Hallberg-NOAA , I added you as a reviewer to PR #72 , which should fix this bug.