Drop the .c suffix of sql files to preprocess

The suffix was added to make compilers happy about preprocessing,
but invoking cpp directly should be just fine (SQLPP macro)

This is an experimental change under postgis/, if everyone is happy
the same thing will be done for raster/ and topology/

git-svn-id: http://svn.osgeo.org/postgis/trunk@11175 b70326c6-7e19-0410-871a-916f4a2858ee