build, encoding: include stdint

The stdint.h is not included when building test, then including this module inside encoding.h

src/include/espeak-ng/encoding.h

@@ -17,6 +17,8 @@
 #ifndef ESPEAK_NG_ENCODING_H
 #define ESPEAK_NG_ENCODING_H
 
+#include <stdint.h>
+
 #ifdef __cplusplus
 extern "C"
 {