external/svox: libexpat now exports its headers.

Change-Id: Id5b4e94bf7a03d3ef80e06cf2bf10700fa1d7319
diff --git a/pico/tts/svox_ssml_parser.h b/pico/tts/svox_ssml_parser.h
index 55708f0..473fed9 100755
--- a/pico/tts/svox_ssml_parser.h
+++ b/pico/tts/svox_ssml_parser.h
@@ -17,7 +17,7 @@
 
 #include <stdio.h>
 #include <stdlib.h>
-#include <libexpat/expat.h>
+#include <expat.h>
 
 #ifndef _SVOX_SSML_PARSER_H_
 #define _SVOX_SSML_PARSER_H_