package/ltrace: fix autoreconf with autoconf >= 2.70

Fix the following build failure raised since bump of autoconf to version
2.71 in commit ecd54b65:

autoreconf: error: cannot create config/autoconf: No such file or
directory

Indeed, the config/autoconf directory does not exist, and therefore
the AC_CONFIG_AUX_DIR invocation serves no purpose.

Fixes:
 - http://autobuild.buildroot.org/results/433599039d6e1ee301465f9867e169c121a0646f



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>