gdbsupport: move xfree into its own file

In the next commit I'd like to reference gdb_unique_ptr within the
common-utils.h file.  However, this requires that I include
gdb_unique_ptr.h, which requires that xfree be defined.

Interestingly, gdb_unique_ptr.h doesn't actually include anything that
defines xfree, but I was finding that when I added a gdb_unique_ptr.h
include to common-utils.h I was getting a dependency cycle; before my
change xfree was defined when gdb_unique_ptr.h was processed, while
after my change it was not, and this made g++ unhappy.

To break this cycle, I propose to move xfree into its own header file,
gdb-xfree.h, which I'll then include into gdb_unique_ptr.h and
common-utils.cc.

gdbsupport/common-utils.cc

@@ -21,6 +21,7 @@
 #include "common-utils.h"
 #include "host-defs.h"
 #include "safe-ctype.h"
+#include "gdbsupport/gdb-xfree.h"
 
 void *
 xzalloc (size_t size)