Makefile: fix 'hdr-check' when GCRYPT not installed

Makefile: fix 'hdr-check' when GCRYPT not installed

If the GCRYPT_SHA256 build variable is not set, then the 'hdr-check'
target complains about the missing <gcrypt.h> header file. Add the
'sha256/gcrypt.h' header file to the exception list, if the build
variable is not defined. While here, replace the 'xdiff%' filter
pattern with 'xdiff/%' (and similarly for the compat pattern) since
the original pattern inadvertently excluded the 'xdiff-interface.h'
header.

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>