dropbear.inc: add dependency on virtual/crypt to fix build with glibc-2.28

dropbear.inc: add dependency on virtual/crypt to fix build with glibc-2.28

configure tests crypt() existence with:

dnl We test for crypt() specially. On Linux (and others?) it resides in libcrypt
dnl but we don't want link all binaries to -lcrypt, just dropbear server.
dnl OS X doesn't need -lcrypt
AC_CHECK_FUNC(crypt, found_crypt_func=here)
AC_CHECK_LIB(crypt, crypt,
        [
        CRYPTLIB="-lcrypt"
        found_crypt_func=here
        ])
AC_SUBST(CRYPTLIB)
if test "t$found_crypt_func" = there; then
AC_DEFINE(HAVE_CRYPT, 1, [crypt() function])
fi

but that silently fails with glibc-2.28 and a bit later do_compile fails with;
http://errors.yoctoproject.org/Errors/Details/185895/

../dropbear-2018.76/sysoptions.h:237:3: error: #error "DROPBEAR_SVR_PASSWORD_AUTH requires `crypt()'."
  #error "DROPBEAR_SVR_PASSWORD_AUTH requires `crypt()'."
   ^~~~~

Add dependency on virtual/crypt so that do_configure detects it correctly.

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>