src: Include internal/syscall.h in stat files

Reference solution for implementing `stat()` and `fstat()` relies on
system call being made. Include `internal/syscall.h` in corresponding
source code files to provide the `syscall()` function.

Signed-off-by: Razvan Deaconescu <razvan.deaconescu@upb.ro>

src/stat/fstat.c

 // SPDX-License-Identifier: BSD-3-Clause
 
 #include <sys/stat.h>
+#include <internal/syscall.h>
 #include <errno.h>
 
 int fstat(int fd, struct stat *st)

src/stat/stat.c

 // SPDX-License-Identifier: BSD-3-Clause
 
 #include <sys/stat.h>
+#include <internal/syscall.h>
 #include <fcntl.h>
 #include <errno.h>