portmap: fix static linking

Portmap builds and links, however, does not get built correctly when
BR2_PREFER_STATIC_LIB is selected.  There are no dynamic libraries in
rootfs.tar, however, portmap gets linked dynamically without regards to
the PREFER_STATIC_LIB tags.  LDFLAGS was not being passed into the build of
portmap.

Signed-off-by: Andy Kennedy <andy.kennedy@adtran.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>