atftp: fix musl compile

- add missing __THROW, __BEGIN_DECLS, __END_DECLS and __attribute_pure__
  defines in atftp own argz.h file

- add missing sys/types.h include in tftp.h and tftpd.h

Fixes [1]:

argz.h:61:1: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'extern'
 extern error_t __argz_create (char *__const __argv[], char **__restrict __argz,
 ^
argz.h: In function 'argz_create':
argz.h:64:34: error: expected declaration specifiers before '__THROW'
        size_t *__restrict __len) __THROW;

[1] http://autobuild.buildroot.net/results/b7f/b7f205ea2066788ea78a1a819f1b592e84e2455e/



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>