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.

---
 meta/recipes-core/dropbear/dropbear.inc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 2e2fbc15af..b634e0e53e 100644
@@ -7,7 +7,7 @@ SECTION = "console/network"
 LICENSE = "MIT & BSD-3-Clause & BSD-2-Clause & PD"
 LIC_FILES_CHKSUM = "file://LICENSE;md5=a5ec40cafba26fc4396d0b550f824e01"
 
-DEPENDS = "zlib"
+DEPENDS = "zlib virtual/crypt"
 RPROVIDES_${PN} = "ssh sshd"
 
 DEPENDS += "${@bb.utils.contains('DISTRO_FEATURES', 'pam', 'libpam', '', d)}"

2.17.1