utils: Use the more low-level __NR_ prefix to refer to the syscall number

The __NR_ constants are also defined in the Android headers.

src/libstrongswan/utils/utils.c

@@ -167,7 +167,7 @@ void closefrom(int low_fd)
 	dir_fd = open("/proc/self/fd", O_RDONLY);
 	if (dir_fd != -1)
 	{
-		while ((len = syscall(SYS_getdents64, dir_fd, buffer,
+		while ((len = syscall(__NR_getdents64, dir_fd, buffer,
 							  sizeof(buffer))) > 0)
 		{
 			for (offset = 0; offset < len; offset += entry->d_reclen)