Permit use of vsprintf() in PostgreSQL code.

The next commit needs it.  Back-patch to 9.0 (all supported versions).

src/port/snprintf.c

@@ -99,6 +99,7 @@
 /* Prevent recursion */
 #undef	vsnprintf
 #undef	snprintf
+#undef	vsprintf
 #undef	sprintf
 #undef	vfprintf
 #undef	fprintf
@@ -178,7 +179,7 @@ pg_snprintf(char *str, size_t count, const char *fmt,...)
 	return len;
 }
 
-static int
+int
 pg_vsprintf(char *str, const char *fmt, va_list args)
 {
 	PrintfTarget target;