[gdb/testsuite] Add missing include in gdb.base/ctf-ptype.c

[gdb/testsuite] Add missing include in gdb.base/ctf-ptype.c

On fedora rawhide, when running test-case gdb.base/ctf-ptype.exp, I get:
...
gdb compile failed, ctf-ptype.c: In function 'main':
ctf-ptype.c:242:29: error: implicit declaration of function 'malloc' \
  [-Wimplicit-function-declaration]
  242 |   v_char_pointer = (char *) malloc (1);
      |                             ^~~~~~
ctf-ptype.c:1:1: note: include '<stdlib.h>' or provide a declaration of 'malloc'
  +++ |+#include <stdlib.h>
    1 | /* This test program is part of GDB, the GNU debugger.
...

Fix this by adding the missing include.

Tested on aarch64-linux.