Missing header from sndio backend.

The sndio backend is currently missing the string.h header which results in the following warnings...

ao_sndio.c: In function 'ao_plugin_set_option':
ao_sndio.c:84: warning: incompatible implicit declaration of built-in function 'strdup'
ao_sndio.c: In function 'ao_plugin_open':
ao_sndio.c:105: warning: incompatible implicit declaration of built-in function 'strdup'
ao_sndio.c:127: warning: incompatible implicit declaration of built-in function 'strdup'