fs: sdcardfs: Declare LOOKUP_CASE_INSENSITIVE unconditionally

Attempts to build sdcardfs as module fail with

fs/sdcardfs/lookup.c: In function '__sdcardfs_lookup':
fs/sdcardfs/lookup.c:243:5: error: 'LOOKUP_CASE_INSENSITIVE' undeclared

This occurs because the define is enclosed with #ifdef
CONFIG_SDCARD_FS_CI_SEARCH. If SDCARD_FS_CI_SEARCH is configured to be
built as module, this does not work. Alternatives would be to use #if
IS_ENABLED(CONFIG_SDCARD_FS_CI_SEARCH), or to declare SDCARD_FS_CI_SEARCH
as bool, but that does not work because the define is used unconditionally
in the source.

Note that LOOKUP_CASE_INSENSITIVE is only set but not evaluated in the
current source code, so setting the flag has no real effect.

Fixes: 84a1b7d3 ("Included sdcardfs source code for kernel 3.0")
Cc: Daniel Rosenberg <drosen@google.com>
Signed-off-by: Guenter Roeck <groeck@chromium.org>