util/log: drop unnecessary _XOPEN_SOURCE

snprintf() is actually ISO C99.

We still need POSIX for clock_gettime().

util/log.c

@@ -1,4 +1,4 @@
-#define _XOPEN_SOURCE 700 // for snprintf
+#define _POSIX_C_SOURCE 200809L
 #include <errno.h>
 #include <stdarg.h>
 #include <stdio.h>