libwebsockets: add patch to fix musl build

libwebsockets: add patch to fix musl build

Add patch from upstream [1] to fix musl build:
fatal error: sys/cdefs.h: No such file or directory

Fixes:
http://autobuild.buildroot.net/results/564/564675be0d4fbfa113a0f2e52874e0dce2d5ba5e/
http://autobuild.buildroot.net/results/c41/c41d87c0cf22f3b88e5f80ad6bf5e81900603c1d/

Backported from: 9eec5e4470397632af035c429295a41b74bd0585

[1] https://github.com/warmcat/libwebsockets/commit/9eec5e4470397632af035c429295a41b74bd0585

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>