Add strict prototypes for functions with no arguments. (#295)

Using compile flags `-Werror -Wstrict-prototypes` (with Clang at least),
the "prototypes" of some functions are flagged as an error. This is
because of the difference between:
```
	int no_args_please(void);
```
and
```
	int no_args_please();
```
Where the former is a strict prototype that says "no arguments are
allowed". With the compile flags mentioned, ReadStat fails to
compile with error messages like this:

```
In file included from src/readstat_bits.c:9:
src/readstat_bits.h:10:29: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
int machine_is_little_endian();
```

This PR adds the missing `void` in a handful of places where Clang
complains about it with those strict compile flags.

src/readstat_bits.h

@@ -7,7 +7,7 @@
 #undef READSTAT_MACHINE_IS_TWOS_COMPLEMENT
 #define READSTAT_MACHINE_IS_TWOS_COMPLEMENT 0
 
-int machine_is_little_endian();
+int machine_is_little_endian(void);
 
 char ones_to_twos_complement1(char num);
 int16_t ones_to_twos_complement2(int16_t num);

src/readstat_variable.c

@@ -2,7 +2,7 @@
 #include <stdlib.h>
 #include "readstat.h"
 
-static readstat_value_t make_blank_value();
+static readstat_value_t make_blank_value(void);
 static readstat_value_t make_double_value(double dval);
 
 static readstat_value_t make_blank_value() {

src/sas/ieee.c

@@ -16,7 +16,7 @@ static void ieee2xpt(unsigned char *ieee, unsigned char *xport);
 
 #ifndef FLOATREP
 #define FLOATREP get_native()
-int get_native();
+int get_native(void);
 #endif
 
 void memreverse(void *intp_void, int l) {

src/spss/readstat_por.h

@@ -31,7 +31,7 @@ typedef struct por_ctx_s {
     ck_hash_table_t *var_dict;
 } por_ctx_t;
 
-por_ctx_t *por_ctx_init();
+por_ctx_t *por_ctx_init(void);
 void por_ctx_free(por_ctx_t *ctx);
 ssize_t por_utf8_encode(const unsigned char *input, size_t input_len, 
         char *output, size_t output_len, uint16_t lookup[256]);