wad: Make OpenFile parameter const

src/w_file.h

@@ -28,7 +28,7 @@ typedef struct _wad_file_s wad_file_t;
 typedef struct
 {
     // Open a file for reading.
-    wad_file_t *(*OpenFile)(char *path);
+    wad_file_t *(*OpenFile)(const char *path);
 
     // Close the specified file.
     void (*CloseFile)(wad_file_t *file);

src/w_file_posix.c

@@ -74,7 +74,7 @@ unsigned int GetFileLength(int handle)
     return lseek(handle, 0, SEEK_END);
 }
    
-static wad_file_t *W_POSIX_OpenFile(char *path)
+static wad_file_t *W_POSIX_OpenFile(const char *path)
 {
     posix_wad_file_t *result;
     int handle;

src/w_file_stdc.c

@@ -30,7 +30,7 @@ typedef struct
 
 extern wad_file_class_t stdc_wad_file;
 
-static wad_file_t *W_StdC_OpenFile(char *path)
+static wad_file_t *W_StdC_OpenFile(const char *path)
 {
     stdc_wad_file_t *result;
     FILE *fstream;

src/w_file_win32.c

@@ -86,7 +86,7 @@ unsigned int GetFileLength(HANDLE handle)
     return result;
 }
    
-static wad_file_t *W_Win32_OpenFile(char *path)
+static wad_file_t *W_Win32_OpenFile(const char *path)
 {
     win32_wad_file_t *result;
     wchar_t wpath[MAX_PATH + 1];