ubifs-utils: link libmissing.a in case execinfo.h isn't present

ubifs-utils: link libmissing.a in case execinfo.h isn't present

On musl execinfo.h doesn't exist, but ubifs-utils uses backtrace() when
reporting errors.  This results in build failures under musl.

Handily, libmissing.a already exists with a stub implementation of
backtrace().

Guard the execinfo.h include and if it isn't available instead include
libmissing.h, and link to libmissing.a to provide backtrace() if needed.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>