btrfs-progs: don't use <execinfo.h> with uClibc

This commit adds a patch to btrfs-progs that fixes the build of
docker-engine, and possibly other packages including kerncompat.h from
btrfs-progs.

For the btrfs-progs build itself, a --disable-backtrace option allows to
indicate whether backtrace()/<execinfo.h> support should be used or
not. However, once btrfs-progs is installed, it simply looks at whether
__GLIBC__ is defined or not to decide to use backtrace() or not.

However, uClibc defines __GLIBC__ but does not provide backtrace()
functionality. The additional patch tweaks the kerncompat.h to look at
__UCLIBC__ and not use backtrace() functionality in this case.

Fixes:

  http://autobuild.buildroot.net/results/9dc9370a79c5c44e6c92be6a44334842c211d923/



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