Add strxfrm_l to list of functions where Windows adds an underscore.

Per buildfarm failure on bowerbird after last night's commit
4ea51cdf.

Peter Geoghegan

src/include/port/win32.h

@@ -341,6 +341,7 @@ typedef int pid_t;
 #define isspace_l _isspace_l
 #define iswspace_l _iswspace_l
 #define strcoll_l _strcoll_l
+#define strxfrm_l _strxfrm_l
 #define wcscoll_l _wcscoll_l
 #define wcstombs_l _wcstombs_l
 #define mbstowcs_l _mbstowcs_l