gdb.base/sizeof.exp C++ify

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.