common: Drop net.h from common header

Move this header out of the common header. Network support is used in
quite a few places but it still does not warrant blanket inclusion.

Note that this net.h header itself has quite a lot in it. It could be
split into the driver-mode support, functions, structures, checksumming,
etc.

Signed-off-by: Simon Glass <sjg@chromium.org>

board/qemu-mips/qemu-mips.c

@@ -6,6 +6,7 @@
 
 #include <common.h>
 #include <command.h>
+#include <net.h>
 #include <asm/mipsregs.h>
 #include <asm/io.h>
 #include <netdev.h>