BUILD: http: remove a warning on strndup

BUILD: http: remove a warning on strndup

The latest commit about set-map/add-acl/... causes this warning for
me :

src/proto_http.c: In function 'parse_http_req_cond':
src/proto_http.c:8863: warning: implicit declaration of function 'strndup'
src/proto_http.c:8863: warning: incompatible implicit declaration of built-in function 'strndup'
src/proto_http.c:8890: warning: incompatible implicit declaration of built-in function 'strndup'
src/proto_http.c:8917: warning: incompatible implicit declaration of built-in function 'strndup'
src/proto_http.c:8944: warning: incompatible implicit declaration of built-in function 'strndup'

Use my_strndup() instead of strndup() which is not portable. No backport
needed.