pgsql: Change PyInit_plpy to external linkage

Change PyInit_plpy to external linkage

Module initialization functions in Python 3 must have external
linkage, because PyMODINIT_FUNC does dllexport on Windows-like
platforms. Without this change, the build with Python 3 fails on
Windows.

Branch
------
REL9_0_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/d4c24254fab0b9908211f990c4239664f3ed5630

Modified Files
--------------
src/pl/plpython/plpython.c | 8 +++++++-
1 files changed, 7 insertions(+), 1 deletions(-)