libs/libc/getpgid: add getpgid implementation

1. the getpgid function can help to pass the
ltp/open_posix_testsuite/killpg related testcases
2. NuttX do not support process group, so we use the process id as
process group id
3. the implementation are referred to: https://pubs.opengroup.org/onlinepubs/9699919799/functions/getpgid.html

Signed-off-by: guoshichao <guoshichao@xiaomi.com>

include/unistd.h

@@ -310,6 +310,7 @@ extern "C"
 pid_t   fork(void);
 pid_t   vfork(void);
 pid_t   getpid(void);
+pid_t   getpgid(pid_t pid);
 pid_t   getpgrp(void);
 pid_t   gettid(void);
 pid_t   getppid(void);