pack-bitmap.h: remove magic number

pack-bitmap.h: remove magic number

When we ran `make hdr-check` with the following patch

	diff --git a/Makefile b/Makefile
	index f879697ea3..d8df4e316b 100644
	--- a/Makefile
	+++ b/Makefile
	@@ -2773,7 +2773,7 @@ CHK_HDRS = $(filter-out $(EXCEPT_HDRS),$(patsubst ./%,%,$(LIB_H)))
	HCO = $(patsubst %.h,%.hco,$(CHK_HDRS))

	$(HCO): %.hco: %.h FORCE
	-	$(QUIET_HDR)$(CC) -include git-compat-util.h -I. -o /dev/null -c -xc $<
	+	$(QUIET_HDR)$(CC) -include git-compat-util.h -I. -o /dev/null -c -xc $(ALL_CFLAGS) $<

	.PHONY: hdr-check $(HCO)
	hdr-check: $(HCO)

and with `DEVELOPER=1`, we got the following warning on Arch Linux:

	pack-bitmap.h:20:19: error: ‘BITMAP_IDX_SIGNATURE’ defined but not used [-Werror=unused-const-variable=]
	   20 | static const char BITMAP_IDX_SIGNATURE[] = {'B', 'I', 'T', 'M'};
	      |                   ^~~~~~~~~~~~~~~~~~~~
	cc1: all warnings being treated as errors

"Use" the BITMAP_IDX_SIGNATURE variable by making the size of
bitmap_disk_header.magic equal to the size of BITMAP_IDX_SIGNATURE,
thereby eliminating the magic number (4).

An alternative was to simply add MAYBE_UNUSED, however that does not
eliminate the magic number.

Another alternative was to change the definition to

	extern const char BITMAP_IDX_SIGNATURE[4];

However, this design was also not chosen as the static definition allows
us to keep the declaration together for readability along with removing
the magic number.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>