pgsql: Fix compiler warnings in ereport messages on mingw.

Log Message:
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Fix compiler warnings in ereport messages on mingw.
ITAGAKI Takahiro

Modified Files:
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pgsql/src/backend/port:
win32_shmem.c (r1.1 -> r1.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/port/win32_shmem.c.diff?r1=1.1&r2=1.2)