mail/exim: Fix build (linker) flags

"-export-dynamic" in EXIM_DYNAMIC_LDFLAGS is a linker option, currently
producing a warning during build:

/usr/bin/ld: warning: cannot find entry symbol xport-dynamic; defaulting to 00000000004019c0

The only reason that plugin modules work is that the option is redundant
with -rdynamic.

This change makes the minimum necessary change not relying on linker
argument compatibility (- vs --).

PR: 236426
Submitted by: <andrew tao11 riddles org uk>
Approved by: portmgr (maintainer timeout: ~6 weeks, "just fix it")
MFH: 2019Q2