Suppress GCC 6 warning about self-comparison

Reviewed-by: Thomas Munro <thomas.munro@enterprisedb.com>

src/port/path.c

@@ -86,7 +86,11 @@ skip_drive(const char *path)
 bool
 has_drive_prefix(const char *path)
 {
+#ifdef WIN32
 	return skip_drive(path) != path;
+#else
+	return false;
+#endif
 }
 
 /*