common/log: bump POSIX version used after e81d9fde66

../common/log.c:63:16: error: use of undeclared identifier 'CLOCK_MONOTONIC'
        clock_gettime(CLOCK_MONOTONIC, &start_time);
                      ^
../common/log.c:75:16: error: use of undeclared identifier 'CLOCK_MONOTONIC'
        clock_gettime(CLOCK_MONOTONIC, &ts);
                      ^

common/log.c

@@ -1,4 +1,4 @@
-#define _POSIX_C_SOURCE 199506L
+#define _POSIX_C_SOURCE 200112L
 #include <signal.h>
 #include <stdarg.h>
 #include <stdio.h>