btrfs-progs: fix build breakage on uClibc without backtrace

uClibc has optional support for backtrace() hence on default
configurations where it's disabled it'll fail.
Add patch to make backtrace support conditional and disable it for
uClibc-based builds since it's mostly for debugging purposes. Fixes:
http://autobuild.buildroot.net/results/325/32523398a8c66a7ac6d3e789332d1b57e090aef1/



Patch status: sent upstream.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>