package/dhcp: add host-gawk optional dependency

package/dhcp: add host-gawk optional dependency

On hosts where gawk is not available, it is not possible to build the
package with server option (BR2_PACKAGE_DHCP_SERVER).
The build goes through without errors but the binaries are not created
and installed. The reason is that autotools cannot find gawk.

Fixes: Bug 13781
Reported-by: Kay Jeschonneck <kay.jeschonneck@airbus.com>
Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>