package/ffmpeg: prevent stripping the ffmpeg binary

Packages shouldn't strip executables by themselves, so that Buildroot
controls whether stripping should occur or not. This also fixes the
build on Blackfin FLAT where stripping actually doesn't work because
stripping FLAT binaries is not supported.

Inspired by Thomas:
http://git.buildroot.net/buildroot/commit/package/hdparm/hdparm.mk?id=11dbb615338cd768acc2c3138064ddf557e9b60b

Fixes
http://autobuild.buildroot.net/results/581/581c43b50c9717e53334eb156ece95fe499ccb04/
http://autobuild.buildroot.net/results/57a/57a1bdc22b803114c018fa007884385bf8355151/
http://autobuild.buildroot.net/results/d8a/d8a1aadebd302abc923bc4865e1242ab2479d641/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>