After the change from PCRE to PCRE2 when enabling POSTFIX_PCRE the
resulting postfix build no longer reports pcre as an available module.
(postfconf -m). This is caused by using the wrong syntax for the
HAS_PCRE build flag (HAS_PCRE2 vs HAS_PCRE=2). This fixes the build flag
to use the correct syntax.
Signed-off-by: Felix Matouschek <felix@matouschek.org>