tests: net: all: Compile test PTP library code

Add PTP library to be compile tested by network all tests.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>

tests/net/all/prj.conf

diff --git a/tests/net/all/prj.conf b/tests/net/all/prj.conf
index 22b6e25..03269e1 100644
--- a/tests/net/all/prj.conf
+++ b/tests/net/all/prj.conf
@@ -320,6 +320,9 @@
 CONFIG_NET_GPTP_SYNC_RECEIPT_TIMEOUT=10
 CONFIG_NET_GPTP_USE_DEFAULT_CLOCK_UPDATE=y
 
+# PTP
+CONFIG_PTP=y
+
 # LLDP
 CONFIG_NET_LLDP_CHASSIS_ID_MAC0=0x01
 CONFIG_NET_LLDP_CHASSIS_ID_MAC1=0x02