package/rtmpdump: Fix static build

Fixes
http://autobuild.buildroot.net/results/04c/04cc4f4878466715d980442bd7114fecbeb51c0b/



Quote from README:

A shared library is now built by default, in addition to the static
library. You can also turn it off if desired

  $ make SHARED=

The rtmpdump programs still link to the static library, regardless.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>