IPA-SAM: Fix build with samba 4.4

samba_util.h is not shipped with samba-4.4
and it was indirectly included by "ndr.h"

Some functions have prototypes in different header file
"util/talloc_stack.h" and other does not have declarations
in other header file. But they are still part of libsamba-util.so

sh$ objdump -T /usr/lib64/libsamba-util.so.0.0.1 | grep -E "trim_s|xstrdup"
0000000000022200 g    DF .text  000000000000001f  SAMBA_UTIL_0.0.1 smb_xstrdup
00000000000223b0 g    DF .text  000000000000019d  SAMBA_UTIL_0.0.1 trim_string

ipa_sam.c: In function 'ldapsam_uid_to_sid':
ipa_sam.c:836:24: warning: implicit declaration of function 'talloc_stackframe'
                  [-Wimplicit-function-declaration]
  TALLOC_CTX *tmp_ctx = talloc_stackframe();
                        ^
ipa_sam.c: In function 'pdb_init_ipasam':
ipa_sam.c:4493:2: warning: implicit declaration of function 'trim_string'
                  [-Wimplicit-function-declaration]
  trim_string( uri, "\"", "\"" );
  ^
ipa_sam.c:4580:26: warning: implicit declaration of function 'smb_xstrdup'
                   [-Wimplicit-function-declaration]
  ldap_state->domain_dn = smb_xstrdup(dn);
                          ^

Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>

daemons/ipa-sam/ipa_sam.c

@@ -19,6 +19,12 @@
 #include <util/data_blob.h>
 #include <util/time.h>
 #include <util/debug.h>
+#include <util/talloc_stack.h>
+
+#ifndef _SAMBA_UTIL_H_
+bool trim_string(char *s, const char *front, const char *back);
+char *smb_xstrdup(const char *s);
+#endif
 
 #include <core/ntstatus.h>
 #include <gen_ndr/security.h>