elfutils: Pass -Wno-unused-result to silence warnings as errors

elfutils turns on -Werror by default, and patch 100-musl-compat.patch
changes how strerror_r is used and we no longer use the function's
return value. This causes the following build error/warning to occur
with glibc-based toolchains:

dwfl_error.c: In function 'dwfl_errmsg':
dwfl_error.c:158:18: error: ignoring return value of 'strerror_r',
declared with attribute warn_unused_result [-Werror=unused-result]
       strerror_r (error & 0xffff, s, sizeof(s));
                  ^
cc1: all warnings being treated as errors

Fixing this would be tricky as there are two possible signatures for
strerror_r (XSI and GNU), just turn off unused-result warnings instead.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>