zebra: add developer test functions for FPM code

Add test functions for the zebra code that interfaces with the
Forwarding Plane Manager. These functions can be invoked in a
development build via the recently-added 'invoke' command.

For example:

  # invoke zebra function zfpm_dt_benchmark_protobuf_encode 100000

Changes:

  * zebra/zebra_fpm_dt.c

    Add the following functions. Each function encodes or decodes a
    route in a particular FPM format a specified number of times.

    - zfpm_dt_benchmark_netlink_encode()
    - zfpm_dt_benchmark_protobuf_encode()
    - zfpm_dt_benchmark_protobuf_decode()

  * zebra/Makefile.am

    Compile zebra_fpm_dt when building a development build.

Signed-off-by: Avneesh Sachdev <avneesh@sproute.com>
diff --git a/zebra/Makefile.am b/zebra/Makefile.am
index ab09a36..abd3797 100644
--- a/zebra/Makefile.am
+++ b/zebra/Makefile.am
@@ -26,6 +26,10 @@
 protobuf_srcs = zebra_fpm_protobuf.c
 endif
 
+if DEV_BUILD
+dev_srcs = zebra_fpm_dt.c
+endif
+
 AM_CFLAGS = $(WERROR)
 
 sbin_PROGRAMS = zebra
@@ -36,7 +40,7 @@
 	zserv.c main.c interface.c connected.c zebra_rib.c zebra_routemap.c \
 	redistribute.c debug.c rtadv.c zebra_snmp.c zebra_vty.c \
 	irdp_main.c irdp_interface.c irdp_packet.c router-id.c zebra_fpm.c \
-	$(othersrc) $(protobuf_srcs)
+	$(othersrc) $(protobuf_srcs) $(dev_srcs)
 
 testzebra_SOURCES = test_main.c zebra_rib.c interface.c connected.c debug.c \
 	zebra_vty.c \