Revert socket.h guards: restore original unguarded includes

The previous fix guarded <sys/socket.h> with #ifdef __linux__, but this
breaks macOS and Windows builds because:
- serial_tcp.c and xplane.c are compiled on ALL SITL platforms
- They require socket functions declared in target.h
- Guarding the declarations makes them unavailable on macOS/Windows
- This causes linker errors instead of fixing the issue

Reverting to the original approach:
- macOS has <sys/socket.h> (it's a POSIX system)
- Windows with Cygwin has <sys/socket.h>
- Only non-POSIX Windows needs different handling

The original code with unguarded includes worked on macOS/Windows
before the SocketCAN driver was added. Need to investigate the actual
root cause of the CI failures without breaking the code on other
platforms.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: daijoubu

src/main/target/SITL/target.h

@@ -27,10 +27,7 @@
 #include <stdint.h>
 #include <stdbool.h>
 #include <stddef.h>
-
-#ifdef __linux__
 #include <sys/socket.h>
-#endif
 
 #include <platform.h>
 
@@ -208,10 +205,7 @@ extern bool lockMainPID(void);
 extern void unlockMainPID(void);
 extern void parseArguments(int argc, char *argv[]);
 extern char *strnstr(const char *s, const char *find, size_t slen);
-
-#ifdef __linux__
 extern int lookupAddress (char *, int, int, struct sockaddr *, socklen_t*);
 
 #define IPADDRESS_PRINT_BUFLEN (INET6_ADDRSTRLEN + 16)
 extern char *prettyPrintAddress(struct sockaddr*, char*, size_t);
-#endif