[gdb/testsuite] Add missing includes in gdb.trace/collection.c

[gdb/testsuite] Add missing includes in gdb.trace/collection.c

On fedora rawhide, with test-case gdb.trace/collection.exp, I get:
...
gdb compile failed, collection.c: In function 'strings_test_func':
collection.c:227:13: error: implicit declaration of function 'malloc' \
  [-Wimplicit-function-declaration]
  227 |   longloc = malloc(500);
      |             ^~~~~~
collection.c:1:1: note: \
  include '<stdlib.h>' or provide a declaration of 'malloc'
  +++ |+#include <stdlib.h>
    1 | /* This testcase is part of GDB, the GNU debugger.

collection.c:228:3: error: implicit declaration of function 'strcpy' \
  [-Wimplicit-function-declaration]
  228 |   strcpy(longloc, ... );
      |   ^~~~~~
collection.c:1:1: note: include '<string.h>' or provide a declaration of \
  'strcpy'
  +++ |+#include <string.h>
    1 | /* This testcase is part of GDB, the GNU debugger.
collection.c:230:8: error: implicit declaration of function 'strlen' \
  [-Wimplicit-function-declaration]
  230 |   i += strlen (locstr);
      |        ^~~~~~
collection.c:230:8: note: include '<string.h>' or provide a declaration of \
  'strlen'
...

Fix this by adding the missing includes.

Tested on aarch64-linux.

Approved-By: John Baldwin <jhb@FreeBSD.org>