package/exim: fix target build on some toolchains

package/exim: fix target build on some toolchains

Building with the Sourcery CodeBench ARM 2014.05 the build fails with this
error:

  >>> exim_dbmbuild utility built

  .../buildroot/output/host/bin/arm-none-linux-gnueabi-gcc -DEXIM_DUMPDB exim_dbutil.c
  exim_dbutil.c: In function 'main':
  exim_dbutil.c:568:1: error: 'for' loop initial declarations are only allowed in C99 mode
   for (uschar * key = dbfn_scan(dbm, TRUE, &cursor);
   ^
  exim_dbutil.c:568:1: note: use option -std=c99 or -std=gnu99 to compile your code
  exim_dbutil.c:630:2: error: 'for' loop initial declarations are only allowed in C99 mode
    for (int i = 1; i <= wait->count; i++)
    ^
  exim_dbutil.c:642:6: error: 'for' loop initial declarations are only allowed in C99 mode
        for (int j = 0; j < MESSAGE_ID_LENGTH; j++)
        ^

Fix by enforcing C99. This completes commit
2c692e81a844b30b4d3161dfd9897b3265bb9279 ("package/exim: fix host build")
to also fix target builds.

Fixes: http://autobuild.buildroot.net/results/6b7e08090f5f0f2627cc3e89b349c2052b6e3116/
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>