gprs_ns2_fr: remove include <linux/if.h>

It's not needed because net/if.h already include
the required parts. Furthermore the linux/if.h generates
a compiler error in combination with net/if.h on older systems
(e.g. debian jessie)

Change-Id: I53650e9c55bb9dd98ba60269025e72673e9f82c1
diff --git a/src/gb/gprs_ns2_fr.c b/src/gb/gprs_ns2_fr.c
index 3cf44ef..a651e1d 100644
--- a/src/gb/gprs_ns2_fr.c
+++ b/src/gb/gprs_ns2_fr.c
@@ -340,7 +340,6 @@
 #ifdef ENABLE_LIBMNL
 
 #include <osmocom/core/mnl.h>
-#include <linux/if.h>
 #include <linux/if_link.h>
 #include <linux/rtnetlink.h>