Removal of #include <HsFFI.h> from template-hsc.h breaks largefile support on 32bit Linux

[John Meacham]

I have similar issues to this in jhc due to its pervasive caching of
compilation results. Basically I must keep track of any potentially
ABI-changing flags and ensure they are consistently passed to every
compilation unit and include them in the signature hash along with the
file contents. I make sure to always pass said flags on the command
line to all the tools, as in -D_FILE_OFFSET_BITS=64 gets passed to
both gcc and hsc2hs rather than relying on a config file.

    John