examples/ip_pipeline: disable build when no epoll

The ip_pipeline example requires the epoll.h header from linux, so
disable building the example if the header cannot be found.

Fixes: 4bbf8e30aa ("examples/ip_pipeline: add CLI interface")
Cc: stable@dpdk.org

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>

examples/ip_pipeline/meson.build

@@ -6,6 +6,7 @@
 # To build this example as a standalone application with an already-installed
 # DPDK instance, use 'make'
 
+build = cc.has_header('sys/epoll.h')
 deps += ['pipeline', 'bus_pci']
 allow_experimental_apis = true
 sources = files(