fix compiler warnings introduced in my previous commit

ext/gd/libgd/gd.h

@@ -193,7 +193,7 @@ typedef struct {
 	/* Font data; array of characters, one row after another.
 		Easily included in code, also easily loaded from
 		data files. */
-	const char *data;
+	char *data;
 } gdFont;
 
 /* Text functions take these. */

ext/gd/libgd/gdfontg.c

@@ -4376,7 +4376,7 @@ gdFont gdFontGiantRep =
   0,
   9,
   15,
-  gdFontGiantData
+  (char*)gdFontGiantData
 };
 
 gdFontPtr gdFontGiant = &gdFontGiantRep;

ext/gd/libgd/gdfontl.c

@@ -4633,7 +4633,7 @@ gdFont gdFontLargeRep =
   0,
   8,
   16,
-  gdFontLargeData
+  (char*)gdFontLargeData
 };
 
 gdFontPtr gdFontLarge = &gdFontLargeRep;

ext/gd/libgd/gdfontmb.c

@@ -3863,7 +3863,7 @@ gdFont gdFontMediumBoldRep =
   0,
   7,
   13,
-  gdFontMediumBoldData
+  (char*)gdFontMediumBoldData
 };
 
 gdFontPtr gdFontMediumBold = &gdFontMediumBoldRep;

ext/gd/libgd/gdfonts.c

@@ -3863,7 +3863,7 @@ gdFont gdFontSmallRep =
   0,
   6,
   13,
-  gdFontSmallData
+  (char*)gdFontSmallData
 };
 
 gdFontPtr gdFontSmall = &gdFontSmallRep;

ext/gd/libgd/gdfontt.c

@@ -2584,7 +2584,7 @@ gdFont gdFontTinyRep =
   0,
   5,
   8,
-  gdFontTinyData
+  (char*)gdFontTinyData
 };
 
 gdFontPtr gdFontTiny = &gdFontTinyRep;