Include headers defining the used types

As libavrdude-avrintel.h uses not only uint32_t and related
types from stdint.h, but also size_t from stddef.h, we also
need to #include the latter into libavrdude.h before
libavrdude.h includes libavrdude-avrintel.h

src/libavrdude.h

@@ -24,6 +24,7 @@
 #include <stdio.h>
 #include <limits.h>
 #include <stdbool.h>
+#include <stddef.h>
 #include <stdint.h>
 
 #ifdef LIBAVRDUDE_INCLUDE_INTERNAL_HEADERS