load-tester: Fix load-tester on platforms where plain char is signed

fgetc() returns an int and EOF is usually -1 so when this gets casted to
a char the result depends on whether `char` means `signed char` or
`unsigned char` (the C standard does not specify it).  If it is unsigned
then its value is 0xff so the comparison with EOF will fail as that is an
implicit signed int.

src/libcharon/plugins/load_tester/load_tester.c

@@ -65,7 +65,7 @@ static FILE* make_connection()
 static int initiate(unsigned int count, unsigned int delay)
 {
 	FILE *stream;
-	char c;
+	int c;
 
 	stream = make_connection();
 	if (!stream)