Fix clock_gettime visibility on Linux with -std=c11

Define _POSIX_C_SOURCE in platform.c on Linux so clock_gettime and
CLOCK_MONOTONIC are declared when compiling with strict C11 mode.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: farkasmark

src/platform.c

@@ -1,3 +1,7 @@
+#if defined(__linux__)
+#define _POSIX_C_SOURCE 200112L
+#endif
+
 #include "platform.h"
 #include <stdio.h>
 #include <stdlib.h>