nic_uio: fix build with freebsd 10

nic_uio: fix build with freebsd 10

Compiling the DPDK under FreeBSD gives the following error due to a
missing include <sys/rwlock.h>.

In file included from nic_uio.c:52:
@/vm/vm_pager.h:126:2: error: implicit declaration of function 'rw_assert' is invalid in C99
      [-Werror,-Wimplicit-function-declaration]
        VM_OBJECT_ASSERT_WLOCKED(object);
        ^
@/vm/vm_object.h:226:2: note: expanded from macro 'VM_OBJECT_ASSERT_WLOCKED'
        rw_assert(&(object)->lock, RA_WLOCKED)
        ^
In file included from nic_uio.c:52:
@/vm/vm_pager.h:126:2: error: use of undeclared identifier 'RA_WLOCKED'
@/vm/vm_object.h:226:29: note: expanded from macro 'VM_OBJECT_ASSERT_WLOCKED'
        rw_assert(&(object)->lock, RA_WLOCKED)
                                   ^
In file included from nic_uio.c:52:
@/vm/vm_pager.h:143:2: error: use of undeclared identifier 'RA_WLOCKED'
        VM_OBJECT_ASSERT_WLOCKED(object);
        ^
@/vm/vm_object.h:226:29: note: expanded from macro 'VM_OBJECT_ASSERT_WLOCKED'
        rw_assert(&(object)->lock, RA_WLOCKED)
                                   ^
In file included from nic_uio.c:52:
@/vm/vm_pager.h:167:2: error: use of undeclared identifier 'RA_WLOCKED'
        VM_OBJECT_ASSERT_WLOCKED(object);
        ^
@/vm/vm_object.h:226:29: note: expanded from macro 'VM_OBJECT_ASSERT_WLOCKED'
        rw_assert(&(object)->lock, RA_WLOCKED)
                                   ^
In file included from nic_uio.c:52:
@/vm/vm_pager.h:190:2: error: use of undeclared identifier 'RA_WLOCKED'
        VM_OBJECT_ASSERT_WLOCKED(m->object);
        ^
@/vm/vm_object.h:226:29: note: expanded from macro 'VM_OBJECT_ASSERT_WLOCKED'
        rw_assert(&(object)->lock, RA_WLOCKED)
                                   ^

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>