Include string.h and stdlib.h at all time.

Even if readline support is disabled, some functions declared in
string.h and stdlib.h are used. (e.g. strlen and exit)

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
Merge pull request #8 from omasanori/fix-include

Include string.h and stdlib.h at all time.