mingw: fix compile error

Declare _POSIX_C_SOURCE before sys/time.h to avoid:

util-time.c: In function 'SCUtcTime':
util-time.c:222:12: error: implicit declaration of function 'gmtime_r'; did you mean 'gmtime_s'? [-Werror=implicit-function-declaration]
  222 |     return gmtime_r(&timep, result);
      |            ^~~~~~~~
      |            gmtime_s
util-time.c:222:12: warning: returning 'int' from a function with return type 'struct tm *' makes pointer from integer without a cast [-Wint-conversion]
  222 |     return gmtime_r(&timep, result);
      |            ^~~~~~~~~~~~~~~~~~~~~~~~
util-time.c: In function 'SCLocalTime':
util-time.c:305:9: error: implicit declaration of function 'localtime_r'; did you mean 'localtime_s'? [-Werror=implicit-function-declaration]
  305 |         localtime_r(&timep, &cached_local_tm[lru]);
      |         ^~~~~~~~~~~
      |         localtime_s
util-time.c:321:56: warning: comparison between pointer and integer
  321 |         if (localtime_r(&timep, &cached_local_tm[lru]) == NULL)
      |                                                        ^~
cc1.exe: some warnings being treated as errors

Tickets: #2994 #3051

src/util-time.c

@@ -50,6 +50,14 @@
  * would be considered timed out.
  */
 
+#ifdef OS_WIN32
+/* for MinGW we need to set _POSIX_C_SOURCE before including
+ * sys/time.h. */
+#ifndef _POSIX_C_SOURCE
+#define _POSIX_C_SOURCE 200809L
+#endif
+#endif
+
 #include "suricata-common.h"
 #include "detect.h"
 #include "threads.h"