Fix build for clang

Remove declaring _POSIX_SOURCE manually. Let the compiler take care of
this on its own, except on Windows where the compiler doesn't take care
of this. Additionally, include a missing header file for the definition
of struct sockaddr_in, which we need before we can allocate the struct.
With these fixes, we can build with Clang 18.1.5, although there are
many warnings.

src/debug/GdbStub.cpp

@@ -14,6 +14,7 @@
 
 #ifndef _WIN32
 #include <sys/socket.h>
+#include <netinet/in.h>
 #include <fcntl.h>
 #include <poll.h>
 #include <signal.h>

src/fatfs/ffsystem.c

@@ -3,8 +3,13 @@
 /* (C)ChaN, 2018                                                          */
 /*------------------------------------------------------------------------*/
 
+#ifdef _WIN32
+/* Windows will not define localtime_r for us unless we explicitly request it
+ * with macros. */
 #define _POSIX_SOURCE
 #define _POSIX_THREAD_SAFE_FUNCTIONS
+#endif
+
 #include <time.h>
 
 #include "ff.h"