vnet: Only build tap on Linux

tap requires some porting on FreeBSD, while we wait for those changes
only build tap on Linux.

Type: improvement
Change-Id: I4361bf43764fdb046c2138d4a2ee5d7efa31bd5a
Signed-off-by: Tom Jones <thj@freebsd.org>
diff --git a/src/vnet/CMakeLists.txt b/src/vnet/CMakeLists.txt
index 3225540..516a5fa 100644
--- a/src/vnet/CMakeLists.txt
+++ b/src/vnet/CMakeLists.txt
@@ -995,6 +995,7 @@
 # tap interface (with virtio backend)
 ##############################################################################
 
+if("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux")
 list(APPEND VNET_SOURCES
   devices/tap/cli.c
   devices/tap/tap.c
@@ -1008,6 +1009,7 @@
 list(APPEND VNET_API_FILES
   devices/tap/tapv2.api
 )
+endif()
 
 ##############################################################################
 # tap interface (with virtio backend)