Makefile: Replace merge-file.h with merge-blobs.h in LIB_H

Commit fa2364ec ("Which merge_file() function do you mean?", 06-12-2012)
renamed the files merge-file.[ch] to merge-blobs.[ch], but forgot to
rename the header file in the definition of the LIB_H macro.

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