syslog-ng: Specify location of libesmtp

If --with-libesmtp is omitted a host tool is invoked to detect the
location of the library and headers, which is obviously the wrong thing
for a cross-compile environment. Explicitly add --with-libesmtp to
resolve this.

Fixes:
http://git.buildroot.net/buildroot/commit/?id=539acb7500588d0a277357cf831c9f25f770578d
http://git.buildroot.net/buildroot/commit/?id=01f41585a78406f6e830d5eaf4413a983812a299



Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>