gdb.base/sizeof.exp C++ify

Fixes:

  src/gdb/testsuite/gdb.base/sizeof.c:54:9: error: cannot initialize a variable of type 'char *' with an lvalue of type 'void *'

... when the testcase is built with a C++ compiler.

gdb/testsuite/ChangeLog:

	* gdb.base/sizeof.c (fill): Add cast.

gdb/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2020-09-17  Pedro Alves  <pedro@palves.net>
+
+	* gdb.base/sizeof.c (fill): Add cast.
+
 2020-09-17  Pedro Alves  <pedro@palves.net>
 
 	* gdb.base/share-env-with-gdbserver.c (main): Add cast.

gdb/testsuite/gdb.base/sizeof.c

@@ -51,7 +51,7 @@ struct {
 static void
 fill (void *buf, long sizeof_buf)
 {
-  char *p = buf;
+  char *p = (char *) buf;
   int i;
   for (i = 0; i < sizeof_buf; i++)
     p[i] = "The quick brown dingo jumped over the layzy dog."[i];