Fix error in reading some shapefiles by exporting Gdal.wkbPointZM

[anowacki]

wkbPointZM is defined in Gdal and used within ogr2GMTdataset
with the expectation that it had been brought into scope of the
parent module with the using Gdal statement. However,
wkbPointZM was not actually exported from Gdal, causing an
error when converting some files.

Add wkbPointZM to the list of exports in Gdal to fix this.

This was found when reading a Shapefile containing sets of
points.

Note that this PR does not contain any tests. I don't really
understand Shapefiles (nor geospatial stuff generally) and am
not able to share the files which cause this problem, so I was
not sure how to write a test for this. However
I am willing to help diagnose in any other way I can.

I also wonder whether other things should be exported from
Gdal, but this change fixes reading of isolated points which is
enough for me for now.

[joa-quim]

Thanks for catch and fix. You are right that other issues like this may still exist but hard to test all because that would need lots of different OGR type files. Many of the functions and constants in gdal.jl are not reexported because they are mainly used only inside the GDAL sub-module files. That was not the case of ogr2GMTdataset, though.