stb_vorbis: use const char* when appropriate

stb_vorbis.c

@@ -211,7 +211,7 @@ extern void stb_vorbis_flush_pushdata(stb_vorbis *f);
 // just want to go ahead and use pushdata.)
 
 #if !defined(STB_VORBIS_NO_STDIO) && !defined(STB_VORBIS_NO_INTEGER_CONVERSION)
-extern int stb_vorbis_decode_filename(char *filename, int *channels, int *sample_rate, short **output);
+extern int stb_vorbis_decode_filename(const char *filename, int *channels, int *sample_rate, short **output);
 #endif
 #if !defined(STB_VORBIS_NO_INTEGER_CONVERSION)
 extern int stb_vorbis_decode_memory(unsigned char *mem, int len, int *channels, int *sample_rate, short **output);
@@ -227,7 +227,7 @@ extern stb_vorbis * stb_vorbis_open_memory(unsigned char *data, int len,
 // this must be the entire stream!). on failure, returns NULL and sets *error
 
 #ifndef STB_VORBIS_NO_STDIO
-extern stb_vorbis * stb_vorbis_open_filename(char *filename,
+extern stb_vorbis * stb_vorbis_open_filename(const char *filename,
                                   int *error, stb_vorbis_alloc *alloc_buffer);
 // create an ogg vorbis decoder from a filename via fopen(). on failure,
 // returns NULL and sets *error (possibly to VORBIS_file_open_failure).
@@ -5022,7 +5022,7 @@ stb_vorbis * stb_vorbis_open_file(FILE *file, int close_on_free, int *error, stb
    return stb_vorbis_open_file_section(file, close_on_free, error, alloc, len);
 }
 
-stb_vorbis * stb_vorbis_open_filename(char *filename, int *error, stb_vorbis_alloc *alloc)
+stb_vorbis * stb_vorbis_open_filename(const char *filename, int *error, stb_vorbis_alloc *alloc)
 {
    FILE *f = fopen(filename, "rb");
    if (f) 
@@ -5277,7 +5277,7 @@ int stb_vorbis_get_samples_short(stb_vorbis *f, int channels, short **buffer, in
 }
 
 #ifndef STB_VORBIS_NO_STDIO
-int stb_vorbis_decode_filename(char *filename, int *channels, int *sample_rate, short **output)
+int stb_vorbis_decode_filename(const char *filename, int *channels, int *sample_rate, short **output)
 {
    int data_len, offset, total, limit, error;
    short *data;