package/samba4: needs host-flex

package/samba4: needs host-flex

host-flex is needed to avoid the following build failure since bump to
version 4.14.3 in commit 7df2611e9e93f9c3efea39bf0b5c217564618a28 due to
https://gitlab.com/samba-team/samba/-/commit/942c0d2128cb8e64a9354dde6bdae82a1c1c3d88

Checking for flex
Checking for program 'flex'                                                                     : not found
Embedded Heimdal build requires flex but it was not found.  Install flex or use --with-system-mitkrb5 or --with-system-heimdalkrb5

Fixes:
 - http://autobuild.buildroot.org/results/b9ed8be51a0eef77d6e48755861ae266c3b9f811

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>