Cmake: restore installing the Avahi service file

During commit 804216a we accidently
dropped the CMAKE setting HAVE_AVAHI, however, this flag controls
the setting to install the avahi service file, so it's no longer
installed.

This fixes it by adding it back.

Signed-off-by: Robin Getz <[email protected]>

Author: rgetz

CMakeLists.txt

@@ -265,6 +265,7 @@ if(WITH_NETWORK_BACKEND)
 	elseif(AVAHI_CLIENT_LIBRARIES AND AVAHI_COMMON_LIBRARIES)
 		message(STATUS "Building with Avahi, a DNS SD implementation")
 		set(HAVE_DNS_SD ON)
+		set(HAVE_AVAHI ON)
 
 		list(APPEND LIBIIO_CFILES dns_sd_avahi.c)
 		set(AVAHI_LIBRARIES ${AVAHI_CLIENT_LIBRARIES} ${AVAHI_COMMON_LIBRARIES})