examples/l2fwd-keepalive: skip meson build if no librt

When librt is not present on a system, processing the meson.build file
for this example application causes an error. Make the library
non-mandatory and just mark the example as unbuildable if it is
not present.

Fixes: 89f0711f9ddf ("examples: build some samples with meson")
Cc: stable@dpdk.org

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

examples/l2fwd-keepalive/meson.build

@@ -7,7 +7,13 @@
 # DPDK instance, use 'make'
 
 allow_experimental_apis = true
-ext_deps += cc.find_library('rt')
+librt = cc.find_library('rt', required: false)
+if not librt.found()
+	build = false
+	subdir_done()
+endif
+
+ext_deps += librt
 deps += 'timer'
 sources = files(
 	'main.c', 'shm.c'