Use int to store the value from GetFileLength.

This fixes the clang warning:

    comparison of unsigned expression < 0 is always false
    [-Wtautological-compare]

Reported by Reef Turner <[email protected]>

src/libespeak-ng/dictionary.c

 	int *pw;
 	int length;
 	FILE *f;
-	unsigned int size;
+	int size;
 	char fname[sizeof(path_home)+20];
 
 	strncpy(dictionary_name, name, 40); // currently loaded dictionary name

src/libespeak-ng/synthdata.c

 	if (!ptr) return EINVAL;
 
 	FILE *f_in;
-	unsigned int length;
+	int length;
 	char buf[sizeof(path_home)+40];
 
 	sprintf(buf, "%s%c%s", path_home, PATHSEP, fname);