Fix build with clang

string_util.h tries to redefine snprintf - which doesn't work if
Bionic #defines snprintf, which it does in a #ifdef __clang__ block

Change-Id: I1d98147b39f86ad309e677c377c97f972c3a0d0c
Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>