package/exim: fix build error during install step

package/exim: fix build error during install step

exim builds some files during the 'make install' step, and these fail with
an error:

  lookups/lf_quote.c:49:3: error: 'for' loop initial declarations are only allowed in C99 mode
     for (int j = 0; j < vlength; j++)
     ^

Fix by passing the -std=c99 here, as it is already passed in the build
step.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>