package/dvblast: fix static linking

dvblast links to -lev, which calls functions from -lm, like floor().

dvblast does not use libtool, so it does not pull in the libev
dependencies when doing a static link.

Fix that by always pulling in -lm after -lev. We can not do it from the
.mk, because libraries passed from the .mk end up at the beginning of
the list.

Fixes:
    http://autobuild.buildroot.org/results/1f0/1f02f84fb926839a1adbe3af457bb909fdc44433/
    http://autobuild.buildroot.org/results/6af/6af3abcdced1cc689a59fe11483f5cdcae0cfd38/


    ...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Julian Scheel <julian@jusst.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>