silence -Wwrite-strings in visualtest

visualtest/src/windows/windows_process.c

@@ -15,7 +15,7 @@
 #if defined(__WIN32__)
 
 void
-LogLastError(char* str)
+LogLastError(const char* str)
 {
     LPVOID buffer;
     DWORD dw = GetLastError();

visualtest/src/windows/windows_screenshot.c

@@ -16,7 +16,7 @@
 #if defined(__WIN32__)
 #include <windows.h>
 
-void LogLastError(char* str);
+void LogLastError(const char* str);
 
 static int img_num;
 static SDL_ProcessInfo screenshot_pinfo;