lib: Include more standard headers for drivers to use

Such as string.h and stdlib.h, to be able to compile on older
toolchains like that of Fedora 27.

libfprint/drivers_api.h

@@ -30,6 +30,8 @@
 
 #include <stdint.h>
 #include <errno.h>
+#include <string.h>
+#include <stdlib.h>
 #include <glib.h>
 #include <libusb.h>