package/ffmpeg: fix fenv build failure on ARCv2 with glibc

package/ffmpeg: fix fenv build failure on ARCv2 with glibc

https://git.buildroot.net/buildroot/commit/toolchain?id=0633eb58a2912328508a76f8fe9875a0d113a880
added glibc support for ARCv2 which lacks fenv.h.

Code format was suggested by Arnout:
http://lists.busybox.net/pipermail/buildroot/2017-November/206631.html

Fixes
http://autobuild.buildroot.net/results/aec/aec300d7d4472421398a24202d6871c965bd1be5/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[Thomas: adjust how the Config.in dependency is expressed.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>