Add extern to *_main global variable declarations in header files.

- Global variables declared in header files without
  the use of the 'extern' keword will result in multiple
  instances of the variable to be created by the compiler
  -- one for each different source file in which the
  the header file is included.  This results in wasted
  memory allocated in the BSS segments as well as
  potentially introducing bugs in the application.

Change-Id: I6ef1790b60a0bd9dd3994f8510723decf258b0cc
Signed-off-by: Dave Wallace <dwallacelf@gmail.com>
diff --git a/src/vnet/devices/netmap/netmap.c b/src/vnet/devices/netmap/netmap.c
index fc49ed6..25a3af7 100644
--- a/src/vnet/devices/netmap/netmap.c
+++ b/src/vnet/devices/netmap/netmap.c
@@ -27,6 +27,8 @@
 #include <vnet/ethernet/ethernet.h>
 #include <vnet/devices/netmap/netmap.h>
 
+netmap_main_t netmap_main;
+
 static u32
 netmap_eth_flag_change (vnet_main_t * vnm, vnet_hw_interface_t * hi,
 			u32 flags)