Fixed warnings with gcc option "-Wwrite-strings".

diff --git a/tools/winegcc/utils.h b/tools/winegcc/utils.h
index c01c57f..053798f 100644
--- a/tools/winegcc/utils.h
+++ b/tools/winegcc/utils.h
@@ -30,13 +30,13 @@
 typedef struct {
     size_t maximum;
     size_t size;
-    char** base;
+    const char** base;
 } strarray;
 
 strarray *strarray_alloc(void);
 void strarray_free(strarray* arr);
-void strarray_add(strarray* arr, char* str);
+void strarray_add(strarray* arr, const char* str);
 
-void spawn(strarray* arr);
+void spawn(const strarray* arr);
 
 extern int verbose;