treewide: be explicit about dependence on gettext.h

Dozens of files made use of gettext functions, without explicitly
including gettext.h.  This made it more difficult to find which files
could remove a dependence on cache.h.  Make C files explicitly include
gettext.h if they are using it.

However, while compat/fsmonitor/fsm-ipc-darwin.c should also gain an
include of gettext.h, it was left out to avoid conflicting with an
in-flight topic.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/fsmonitor/fsm-path-utils-darwin.c

@@ -1,5 +1,6 @@
 #include "fsmonitor.h"
 #include "fsmonitor-path-utils.h"
+#include "gettext.h"
 #include <dirent.h>
 #include <errno.h>
 #include <fcntl.h>