CYGWIN: fix build.

The PR fixes these build errors under CYGWIN:

libssh-0.11.0/src/misc.c: In function ‘ssh_strerror’:
libssh-0.11.0/src/misc.c:1984:8: error: assignment to ‘int’ from ‘char *’ makes integer from pointer without a cast [-Werror=int-conversion]
 1984 |     rv = strerror_r(err_num, buf, buflen);
      |        ^

libssh-0.11.0/src/ttyopts.c: In function ‘encode_termios_opts’:
libssh-0.11.0/src/ttyopts.c:267:26: error: ‘PENDIN’ undeclared (first use in this function)
  267 |     SSH_ENCODE_LOCAL_OPT(PENDIN)
      |                          ^~~~~~

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTING.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code