net: sparx5: Add #include to remove warning

main.h uses NUM_TARGETS from main_regs.h, but
the missing include never causes any errors
because everywhere main.h is (currently)
included, main_regs.h is included before.
But since it is dependent on main_regs.h
it should always be included.

Signed-off-by: Casper Andersson <casper.casan@gmail.com>
Reviewed-by: Joacim Zetterling <joacim.zetterling@westermo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>