environment.c: Fix an sparse "symbol not declared" warning

In particular, sparse issues the following warning:

    environment.c:62:5: warning: symbol 'merge_log_config' was not \
        declared. Should it be static?

In order to supress the warning, we include the "fmt-merge-msg.h"
header file, since it contains an appropriate extern declaration for
the 'merge_log_config' variable.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

environment.c

@@ -9,6 +9,7 @@
  */
 #include "cache.h"
 #include "refs.h"
+#include "fmt-merge-msg.h"
 
 char git_default_email[MAX_GITNAME];
 char git_default_name[MAX_GITNAME];