libamcodec: fix build with musl

This commit adds a patch to the libamcodec in order to fix the build
with musl, which was failing due to missing <stdint.h> includes.

Fixes:

  http://autobuild.buildroot.net/results/1f3b0d483603fb69765c15c7189ff975b72d09c3/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>