systemd: add option for valgrind header checking

systemd: add option for valgrind header checking

Add option so that valgrind header checking is controlled. Otherwise,
we would get the following error.

   src/libsystemd/sd-bus/bus-control.c: fatal error: valgrind/mencheck.h: No such file or directory

The above error could be reproduced by the following command.

bitbake valgrind && bitbake systemd -ccleansstate && bitbake systemd -cconfigure
&& bitbake valgrind -ccleansstate && bitbake systemd -ccompile

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>