pgsql: pgwin32_is_junction's argument should be "const char *" not "cha

pgwin32_is_junction's argument should be "const char *" not "char *".

We're passing const strings to it in places, and that's not an
unreasonable thing to do.  Per buildfarm (noted on frogmouth
in particular).

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/06f5fd2f4f3d0991af07456e7cf6632b4ad9d7e7

Modified Files
--------------
src/include/port.h | 2 +-
src/port/dirmod.c  | 4 ++--
2 files changed, 3 insertions(+), 3 deletions(-)