build: remove hardcoded samba include path
    
    SMBCLIENT_FLAGS already provides the samba-4.0 include path. Remove the
    hardcoded samba include path which causes cross-compilation warnings
    such as:
    
    cc1: warning: include location "/usr/include/samba-4.0" is unsafe for
    cross-compilation [-Wpoison-system-directories]
    
    These warnings are treated as errors on build systems such as
    OpenEmbedded.
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>