Fix build for clang

Remove declaring POSIX_SOURCE manually. Let the compiler take care of
this on its own. 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.

Author: Patater

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 +3,6 @@
 /* (C)ChaN, 2018                                                          */
 /*------------------------------------------------------------------------*/
 
-#define _POSIX_SOURCE
-#define _POSIX_THREAD_SAFE_FUNCTIONS
 #include <time.h>
 
 #include "ff.h"