enb: Add modifiers for num_prb

Change-Id: I0b8c834f1c67cc4bf4d07d03358ba2012331cbde
diff --git a/example/scenarios/mod-enb-nprb75.conf b/example/scenarios/mod-enb-nprb75.conf
new file mode 100644
index 0000000..8d79840
--- /dev/null
+++ b/example/scenarios/mod-enb-nprb75.conf
@@ -0,0 +1,3 @@
+modifiers:
+  enb:
+  - num_prb: 75