imap-send: add missing "strbuf.h" include under NO_CURL

imap-send: add missing "strbuf.h" include under NO_CURL

Building with NO_CURL is currently broken since imap-send.c uses things
defined in "strbuf.h" wihtout including it.

The inclusion of that header was removed in eea0e59ffb (treewide: remove
unnecessary includes in source files, 2023-12-23), which failed to
notice that "strbuf.h" was transitively included in imap-send.c via
"http.h", but only if USE_CURL_FOR_IMAP_SEND is defined. Add back the
missing include. Note that it was explicitely added in 3307f7dde2
(imap-send: include strbuf.h, 2023-05-17) after a similar breakage in
ba3d1c73da (treewide: remove unnecessary cache.h includes, 2023-02-24) -
see the thread starting at [1].

It can be verified by inspection that this is the only case where a
header we include is dependent on a Makefile knob in the files modified
in eea0e59ffb.

[1] https://lore.kernel.org/git/20230517070632.71884-1-list@eworm.de/

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>